glamor: Introduce a central place for our pixmap format/type handling.

We had various helper functions trying to come up with the
internalformat/format/type/render formats for pixmaps, and it's much
nicer to just detect what those should be once at startup.  This gives
us a chance to do the right thing for GLES.

It also, notably, fixes our format/type for depth 15 and 16 setup for
desktop GL, so that we actually allocate 16bpp (GL_RGB/565) on most
drivers instead of 32bpp (GL_RGB/UBYTE).

GLES still has regressions over desktop (2 regressions in llvmpipe
XTS, many in rendercheck), but I think this is a good baseline.

Signed-off-by: Eric Anholt <eric@anholt.net>
3 jobs for glamor-formats in 4 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #244941
debian-testing

00:00:34

 
  Build And Test
passed #244942
autotools-build-and-test

00:03:59

passed #244943
meson-build-and-test

00:01:11