altivec yuv->rgb converter

orginal patch  by (Marc Hoffman <mmh at pleasantst dot com>)
critical fixes by (Reza Jelveh <reza.jelveh at tu-harburg dot de>)

known bugs/issues, which should be fixed ASAP by someone who has a ppc:
0..255 vs. 16..235
unneeded recalculation of tables
general cleaup, like removing double initalizing of variables

Originally committed as revision 12699 to svn://svn.mplayerhq.hu/mplayer/trunk/postproc