Test a blit shader implementation

assets/blit.slang

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+// Prism fullscreen texture blit shader.
+//
+// Draws a selected texture from the bindless array as a fullscreen quad that
+// is letterboxed/pillarboxed to preserve aspect ratio (contain-fit). The NDC
+// content rect is supplied via push constants so the texture is never
+// stretched, squashed, or cropped.
+
+struct BlitData {
+	float4 rect;   // NDC fit rect: x0, y0, x1, y1
+	uint   selected;
+	uint   pad[3];
+};
+
+[[vk::push_constant]]
+BlitData blit;
+
+Sampler2D textures[];
+
+struct VSOutput {
+	float4 Pos : SV_POSITION;
+	float2 UV;
+};
+
+[shader("vertex")]
+VSOutput main(uint vertexIndex : SV_VertexID) {
+	VSOutput output;
+	float2 uv = float2(float(vertexIndex & 1), float((vertexIndex >> 1) & 1));
+	output.UV = uv;
+	float2 pos = lerp(blit.rect.xy, blit.rect.zw, uv);
+	output.Pos = float4(pos, 0.0, 1.0);
+	return output;
+}
+
+[shader("fragment")]
+float4 main(VSOutput input) {
+	return textures[NonUniformResourceIndex(blit.selected)].Sample(input.UV);
+}