WIP: Metal shader backend para macOS

- Shaders MSL portados desde GLSL (vertex + fragment)
- Estructura básica de MetalShader class
- Device, command queue, pipeline y buffers creados
- CMakeLists.txt actualizado con Metal frameworks
- assets.txt incluye shaders .metal como opcionales

PENDIENTE:
- Implementar render() loop completo
- Obtener MTLTexture desde SDL_Texture
- Crear sampler state
- Testing en macOS real

Ver METAL_BACKEND_NOTES.md para detalles de implementación.

data/shaders/crtpi_fragment.metal

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+//
+// CRT-Pi Fragment Shader - Metal Shading Language
+// Portado desde GLSL a MSL para macOS
+//
+
+#include <metal_stdlib>
+using namespace metal;
+
+// Configuración (equivalente a los #define en GLSL)
+constant bool SCANLINES = true;
+constant bool MULTISAMPLE = true;
+constant bool GAMMA = true;
+constant bool FAKE_GAMMA = false;
+constant bool CURVATURE = false;
+constant bool SHARPER = false;
+constant int MASK_TYPE = 2;  // 0=none, 1=green/magenta, 2=trinitron
+
+constant float CURVATURE_X = 0.05;
+constant float CURVATURE_Y = 0.1;
+constant float MASK_BRIGHTNESS = 0.80;
+constant float SCANLINE_WEIGHT = 6.0;
+constant float SCANLINE_GAP_BRIGHTNESS = 0.12;
+constant float BLOOM_FACTOR = 3.5;
+constant float INPUT_GAMMA = 2.4;
+constant float OUTPUT_GAMMA = 2.2;
+
+// Estructura de entrada (salida del vertex shader)
+struct VertexOut {
+    float4 position [[position]];
+    float2 texCoord;
+    float filterWidth;
+    // float2 screenScale;  // Solo si CURVATURE está activo
+};
+
+// Uniforms
+struct Uniforms {
+    float2 textureSize;
+};
+
+// Función para calcular el peso de la scanline
+float CalcScanLineWeight(float dist) {
+    return max(1.0 - dist * dist * SCANLINE_WEIGHT, SCANLINE_GAP_BRIGHTNESS);
+}
+
+// Función para calcular scanline con multisampling
+float CalcScanLine(float dy, float filterWidth) {
+    float scanLineWeight = CalcScanLineWeight(dy);
+    if (MULTISAMPLE) {
+        scanLineWeight += CalcScanLineWeight(dy - filterWidth);
+        scanLineWeight += CalcScanLineWeight(dy + filterWidth);
+        scanLineWeight *= 0.3333333;
+    }
+    return scanLineWeight;
+}
+
+// Entry point del fragment shader
+fragment float4 fragment_main(VertexOut in [[stage_in]],
+                              texture2d<float> colorTexture [[texture(0)]],
+                              constant Uniforms& uniforms [[buffer(1)]],
+                              sampler textureSampler [[sampler(0)]]) {
+
+    float2 texcoord = in.texCoord;
+
+    // Si CURVATURE estuviera activo, aquí iría la función Distort()
+
+    float2 texcoordInPixels = texcoord * uniforms.textureSize;
+
+    float2 tc;
+    float scanLineWeight;
+
+    if (!SHARPER) {
+        // Modo normal (no SHARPER)
+        float tempY = floor(texcoordInPixels.y) + 0.5;
+        float yCoord = tempY / uniforms.textureSize.y;
+        float dy = texcoordInPixels.y - tempY;
+        scanLineWeight = CalcScanLine(dy, in.filterWidth);
+        float signY = sign(dy);
+        dy = dy * dy;
+        dy = dy * dy;
+        dy *= 8.0;
+        dy /= uniforms.textureSize.y;
+        dy *= signY;
+        tc = float2(texcoord.x, yCoord + dy);
+    } else {
+        // Modo SHARPER
+        float2 tempCoord = floor(texcoordInPixels) + 0.5;
+        float2 coord = tempCoord / uniforms.textureSize;
+        float2 deltas = texcoordInPixels - tempCoord;
+        scanLineWeight = CalcScanLine(deltas.y, in.filterWidth);
+        float2 signs = sign(deltas);
+        deltas.x *= 2.0;
+        deltas = deltas * deltas;
+        deltas.y = deltas.y * deltas.y;
+        deltas.x *= 0.5;
+        deltas.y *= 8.0;
+        deltas /= uniforms.textureSize;
+        deltas *= signs;
+        tc = coord + deltas;
+    }
+
+    // Muestrear textura (texture() en GLSL = sample() en MSL)
+    float3 colour = colorTexture.sample(textureSampler, tc).rgb;
+
+    if (SCANLINES) {
+        if (GAMMA) {
+            if (FAKE_GAMMA) {
+                colour = colour * colour;
+            } else {
+                colour = pow(colour, float3(INPUT_GAMMA));
+            }
+        }
+        scanLineWeight *= BLOOM_FACTOR;
+        colour *= scanLineWeight;
+
+        if (GAMMA) {
+            if (FAKE_GAMMA) {
+                colour = sqrt(colour);
+            } else {
+                colour = pow(colour, float3(1.0 / OUTPUT_GAMMA));
+            }
+        }
+    }
+
+    // Aplicar máscara CRT
+    if (MASK_TYPE == 0) {
+        return float4(colour, 1.0);
+    } else if (MASK_TYPE == 1) {
+        // Máscara verde/magenta
+        float whichMask = fract(in.position.x * 0.5);
+        float3 mask;
+        if (whichMask < 0.5) {
+            mask = float3(MASK_BRIGHTNESS, 1.0, MASK_BRIGHTNESS);
+        } else {
+            mask = float3(1.0, MASK_BRIGHTNESS, 1.0);
+        }
+        return float4(colour * mask, 1.0);
+    } else if (MASK_TYPE == 2) {
+        // Máscara trinitron
+        float whichMask = fract(in.position.x * 0.3333333);
+        float3 mask = float3(MASK_BRIGHTNESS, MASK_BRIGHTNESS, MASK_BRIGHTNESS);
+        if (whichMask < 0.3333333) {
+            mask.x = 1.0;
+        } else if (whichMask < 0.6666666) {
+            mask.y = 1.0;
+        } else {
+            mask.z = 1.0;
+        }
+        return float4(colour * mask, 1.0);
+    }
+
+    return float4(colour, 1.0);
+}

data/shaders/crtpi_vertex.metal

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+//
+// CRT-Pi Vertex Shader - Metal Shading Language
+// Portado desde GLSL a MSL para macOS
+//
+
+#include <metal_stdlib>
+using namespace metal;
+
+// Estructura de entrada del vertex shader (desde el buffer de vértices)
+struct VertexIn {
+    float2 position [[attribute(0)]];  // Posición del vértice
+    float2 texCoord [[attribute(1)]];  // Coordenadas de textura
+};
+
+// Estructura de salida del vertex shader (entrada al fragment shader)
+struct VertexOut {
+    float4 position [[position]];      // Posición en clip space
+    float2 texCoord;                   // Coordenadas de textura
+    float filterWidth;                 // Ancho del filtro calculado
+    // float2 screenScale;             // Solo si CURVATURE está activo
+};
+
+// Uniforms (constantes del shader)
+struct Uniforms {
+    float2 textureSize;                // Tamaño de la textura (width, height)
+};
+
+// Entry point del vertex shader
+vertex VertexOut vertex_main(VertexIn in [[stage_in]],
+                             constant Uniforms& uniforms [[buffer(1)]]) {
+    VertexOut out;
+
+    // Posición del vértice (ya está en espacio de clip [-1, 1])
+    out.position = float4(in.position, 0.0, 1.0);
+
+    // Pasar coordenadas de textura (invertir Y para SDL, igual que en GLSL)
+    out.texCoord = float2(in.texCoord.x, 1.0 - in.texCoord.y) * 1.0001;
+
+    // Calcular filterWidth dinámicamente basándose en la altura de la textura
+    out.filterWidth = (768.0 / uniforms.textureSize.y) / 3.0;
+
+    // Si CURVATURE estuviera activo:
+    // out.screenScale = float2(1.0, 1.0);
+
+    return out;
+}