CCEffectContrast - Fix for negative contrast values

Multiply the constant offset value, 0.5, by the input alpha value to make sure it is
also in premultiplied alpha space as the RGB values are. Without this correction,
input RGB values of (0,0,0) can become non-(0,0,0) when the contrast adjustment
is less than 1.0.

Author: Thayer J Andrews

cocos2d/CCEffectContrast.m

@@ -54,7 +54,8 @@ -(void)buildFragmentFunctions
     CCEffectFunctionInput *input = [[CCEffectFunctionInput alloc] initWithType:@"vec4" name:@"inputValue" initialSnippet:CCEffectDefaultInitialInputSnippet snippet:CCEffectDefaultInputSnippet];
 
     NSString* effectBody = CC_GLSL(
-                                   return vec4(((inputValue.rgb - vec3(0.5)) * vec3(u_contrast) + vec3(0.5)), inputValue.a);
+                                   vec3 offset = vec3(0.5) * inputValue.a;
+                                   return vec4(((inputValue.rgb - offset) * vec3(u_contrast) + offset), inputValue.a);
                                    );
 
     CCEffectFunction* fragmentFunction = [[CCEffectFunction alloc] initWithName:@"contrastEffect" body:effectBody inputs:@[input] returnType:@"vec4"];