Make drawing to and from RGBA8888 images faster

After profiling drawing with RGBA8888 images most of the time appears
to be spend in converting to and from ARGB32PM.

This patch adds four small converters that are 3-4 times faster than the
generic converter.

Change-Id: I3c7498756f440ca3ea9c1417b26dd8e1953b9d06
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>