gdi/epng, gdi/gpixmap: A few minor optimizations, use argb()

Move "if" outside loop.
Use the cheap argb() function instead of re-doing it the hard way.
Add box-specific display support, in particular vuduo2

When a skin_display_(boxtype).xml exists, use that instead of the
default one. Create a bit of a vfd skin for the vuduo2 (which has
a 140x32 sized vfd).

Could be replaced with a "displayname" configure parameter, so
that a display skin is chosen based on machine features. Then the
--with-textlcd and similar options can be dropped.
Merge remote-tracking branch 'pli/master'

skip display skin commits.