Desktop: Fix Vello rendering colors with doubly-applied gamma by converting to linear in the shader (#2998)

* Fix gamma issue by applying tone mapping in the shader

Vello renders it's values to an RgbaUnorm texture
but if we try to use this in the main rendering
pipeline which renders to an Srgb surface gamma
mapping is applied twice.

* Add comment explaining why we do this to the shader

* Rename variables

Author: TrueDoctor

desktop/src/render/composite_shader.wgsl

@@ -49,17 +49,29 @@ fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
 		return ui;
 	}
 
-	let vp_cord = (in.tex_coords - constants.viewport_offset) * constants.viewport_scale;
+	let viewport_coordinate = (in.tex_coords - constants.viewport_offset) * constants.viewport_scale;
 
-	let ov = textureSample(t_overlays, s_diffuse, vp_cord);
-	let vp = textureSample(t_viewport, s_diffuse, vp_cord);
+	// Vello renders its values to an `RgbaUnorm` texture, but if we try to use this in the main rendering pipeline
+	// which renders to an `Srgb` surface, gamma mapping is applied twice. This converts back to linear to compensate.
+	let overlay_raw = textureSample(t_overlays, s_diffuse, viewport_coordinate);
+	let overlay = vec4<f32>(srgb_to_linear(overlay_raw.rgb), overlay_raw.a);
+	let viewport_raw = textureSample(t_viewport, s_diffuse, viewport_coordinate);
+	let viewport = vec4<f32>(srgb_to_linear(viewport_raw.rgb), viewport_raw.a);
 
-	if (ov.a < 0.001) {
-		return blend(ui, vp);
+	if (overlay.a < 0.001) {
+		return blend(ui, viewport);
 	}
 
-	let comp = blend(ov, vp);
-	return blend(ui, comp);
+	let composite = blend(overlay, viewport);
+	return blend(ui, composite);
+}
+
+fn srgb_to_linear(srgb: vec3<f32>) -> vec3<f32> {
+	return select(
+		pow((srgb + 0.055) / 1.055, vec3<f32>(2.4)),
+		srgb / 12.92,
+		srgb <= vec3<f32>(0.04045)
+	);
 }
 
 fn blend(fg: vec4<f32>, bg: vec4<f32>) -> vec4<f32> {