Proper dither() noise factoring

webrender/res/prim_shared.glsl

@@ -763,8 +763,9 @@ vec4 dither(vec4 color) {
     const int matrix_mask = 7;
 
     ivec2 pos = ivec2(gl_FragCoord.xy) & ivec2(matrix_mask);
-    float noise_factor = 4.0 / 255.0;
-    float noise = texelFetch(sDither, pos, 0).r * noise_factor;
+    float noise_normalized = (texelFetch(sDither, pos, 0).r * 255.0 + 0.5) / 64.0;
+    float noise = (noise_normalized - 0.5) / 256.0; // scale down to the unit length
+
     return color + vec4(noise, noise, noise, 0);
 }
 #endif //WR_FRAGMENT_SHADER