Add source for screen space dither

crates/bevy_pbr/src/render/pbr.wgsl

@@ -24,12 +24,12 @@ struct FragmentInput {
 #endif
 };
 
+// Source: Advanced VR Rendering, GDC 2015, Alex Vlachos, Valve, Slide 49
+// https://media.steampowered.com/apps/valve/2015/Alex_Vlachos_Advanced_VR_Rendering_GDC2015.pdf
 fn screen_space_dither(frag_coord: vec2<f32>) -> vec4<f32> {
-	// Iestyn's RGB dither (7 asm instructions) from Portal 2 X360, slightly modified for VR.
-    var dither = vec3<f32>(dot(vec2<f32>(171.0, 231.0), frag_coord));
-    dither = fract(dither.rgb / vec3<f32>(103.0, 71.0, 97.0));
-	// Subtract 0.5 to avoid slightly brightening the whole viewport.
-    return vec4<f32>((dither.rgb - 0.5) / 255.0, 0.0);
+    var dither = vec3<f32>(dot(vec2<f32>(171.0, 231.0), frag_coord)).xxx;
+    dither = fract(dither.rgb / vec3<f32>(103.0, 71.0, 97.0));// - 0.5;
+    return vec4<f32>(dither.rgb / 255.0, 0.0);
 }
 
 [[stage(fragment)]]