Improve Environment color adjustments; specifically brightness and HDR 2D contrast.

This commit changes adjustments to behave as follows for all rendering configurations:

- Apply brightness to linear-encoded values, preventing contrast, saturation, and hue from being affected.
- Apply contrast to perceptually uniform (nonlinear sRGB-encoded) values, matching existing behavior when HDR 2D is disabled and producing optimal visual quality.
- Apply saturation with even color channel weights. This causes brightness of certain colors to change, but matches existing behavior when HDR 2D is disabled.

Adjustments are applied after glow and tonemapping to match existing behavior.

drivers/gles3/shaders/effects/post.glsl

@@ -86,16 +86,6 @@ vec3 apply_color_correction(vec3 color) {
 #endif // USE_1D_LUT
 #endif // USE_COLOR_CORRECTION
 
-#ifdef USE_BCS
-vec3 apply_bcs(vec3 color) {
-	color = mix(vec3(0.0), color, brightness);
-	color = mix(vec3(0.5), color, contrast);
-	color = mix(vec3(dot(vec3(1.0), color) * 0.33333), color, saturation);
-
-	return color;
-}
-#endif
-
 in vec2 uv_interp;
 
 layout(location = 0) out vec4 frag_color;
@@ -140,12 +130,33 @@ void main() {
 #endif // USE_GLOW
 
 	color.rgb = srgb_to_linear(color.rgb);
+
 	color.rgb = apply_tonemapping(color.rgb, white);
-	color.rgb = linear_to_srgb(color.rgb);
 
 #ifdef USE_BCS
-	color.rgb = apply_bcs(color.rgb);
-#endif
+	// Apply brightness:
+	// Apply to relative luminance. This ensures that the hue and saturation of
+	// colors is not affected by the adjustment, but requires the multiplication
+	// to be performed on linear-encoded values.
+	color.rgb = color.rgb * brightness;
+
+	color.rgb = linear_to_srgb(color.rgb);
+
+	// Apply contrast:
+	// By applying contrast to RGB values that are perceptually uniform (nonlinear),
+	// the darkest values are not hard-clipped as badly, which produces a
+	// higher quality contrast adjustment and maintains compatibility with
+	// existing projects.
+	color.rgb = mix(vec3(0.5), color.rgb, contrast);
+
+	// Apply saturation:
+	// By applying saturation adjustment to nonlinear sRGB-encoded values with
+	// even weights the preceived brightness of blues are affected, but this
+	// maintains compatibility with existing projects.
+	color.rgb = mix(vec3(dot(vec3(1.0), color.rgb) * (1.0 / 3.0)), color.rgb, saturation);
+#else
+	color.rgb = linear_to_srgb(color.rgb);
+#endif // USE_BCS
 
 #ifdef USE_COLOR_CORRECTION
 	color.rgb = apply_color_correction(color.rgb);