Added color filter

Use case: Set all the pixels of an image to a color, for generating
fast background colors.
Allow matching existing alpha value in color generator filter
Put solid color generator overloads back, improve performance
GLSL fixes
Merge pull request #1024 from jjxtra/master

Solid color generator filter alpha option
Merge branch 'master' of git://github.com/BradLarson/GPUImage