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.
Merge pull request #991 from thayerandrews/develop

CCEffectContrast - Fix for negative contrast values