Remove code duplication in sws_rgb2rgb_init.

The lists of assignments of all rgbxtoy variants are removed from
sws_rgb2rgb_init and replaced by a single list inside the template file.
This way, multiple lists are generated by the preprocessor and each list
gets its own initialization function. Those are called from the main
sws_rgb2rgb_init function, if applicable.

Originally committed as revision 22978 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale