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.
3 jobs for glamor-fix-xts-for-gles in 4 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #322527
debian-testing

00:00:28

 
  Build And Test
passed #322528
autotools-build-and-test

00:03:54

passed #322529
meson-build-and-test

00:01:24