[GPU] Allow kMulRGBByAlpha_InConfigFlag and kSwapRAndB_InConfigFlag to both be set

[bsalomon, 2011-11-22 19:45:57]

This was a dumb mistake on my part. It's not caught by testing because we test
on desktop GL where the ARB_texture_swizzle extension allows us to swizzle the
channels via glTexParameter rather than in the shader.

[TomH, 2011-11-22 19:50:29]

LGTM

[bsalomon, 2011-11-22 19:57:42]

Closed with r2733.