Fix upload of glyphs when using RGB32 masks on OpenGL ES

OpenGL ES doesn't allow internal format to be different
from external format, so always do the conversion from
BGRA -> RGBA. We are anyway iterating over all the
pixels so the performance impact of this should be
minimal.

Change-Id: Ie891665ad66e31692b69db02d34be8d303a7d631
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>