glamor: Fix more fallback paths with non-GXcopy rop with GLES

Desktop GL can handle arbitrary rops here, GLES can't. The switch
statement attempts to optimize some cases that GLES can still handle if
we precompute the right pixel value, but we were then throwing that
pixel value away and using gc->fgPixel anyway. Fix this, and now
xts-render passes against glamor+gles.
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