Add dithering (requires OpenGL 2.1 or above)

git-svn-id: https://www.williamfeely.info/svn/dxgl@525 8a90861a-4eca-46d5-b744-240ff16d0c4d
Don't dither the alpha channel.

git-svn-id: https://www.williamfeely.info/svn/dxgl@526 8a90861a-4eca-46d5-b744-240ff16d0c4d