Add SDL3 GPU backend (Vulkan/Metal) with OpenGL fallback

shaders/_common/passthrough.vert.msl

@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+using namespace metal;
+
+// Shared fullscreen-triangle vertex shader for the SDL3 GPU backend.
+// Emits uv in Shadertoy convention: (0,0) bottom-left, (1,1) top-right.
+
+struct PassthroughVOut {
+    float4 pos [[position]];
+    float2 uv;
+};
+
+vertex PassthroughVOut passthrough_vs(uint vid [[vertex_id]]) {
+    const float2 positions[3] = { {-1.0, -1.0}, {3.0, -1.0}, {-1.0, 3.0} };
+    PassthroughVOut out;
+    float2 pos = positions[vid];
+    out.uv = pos * 0.5 + 0.5;
+    out.pos = float4(pos.x, -pos.y, 0.0, 1.0);
+    return out;
+}