bytes when converting between RGB32 variants.

In particular fix the argb -> rgba and abgr -> bgra conversions.

See the thread:
Subject: [FFmpeg-devel] [RFC] RGB32 / BGR32 ethernal bug
Date: Tue, 26 Jan 2010 01:06:18 +0100

Originally committed as revision 30501 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale