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

shaders/_common/passthrough.vk.glsl

@@ -0,0 +1,18 @@
+#version 450
+
+// Shared fullscreen-triangle vertex shader for the SDL3 GPU backend.
+// Emits vUV in Shadertoy convention: (0,0) bottom-left, (1,1) top-right.
+// Y flipped in NDC because Vulkan/Metal point Y down by default.
+
+layout(location = 0) out vec2 vUV;
+
+void main() {
+    const vec2 positions[3] = vec2[3](
+        vec2(-1.0, -1.0),
+        vec2( 3.0, -1.0),
+        vec2(-1.0,  3.0)
+    );
+    vec2 pos = positions[gl_VertexIndex];
+    vUV = pos * 0.5 + 0.5;
+    gl_Position = vec4(pos.x, -pos.y, 0.0, 1.0);
+}