fixing the lamest bug in swscale, all the rgb/bgr->* code did 2x2 downsampling...

fixing the lamest bug in swscale, all the rgb/bgr->* code did 2x2 downsampling for chroma, it should just be 2x1 (the rest of the code also belived its 2x1 ...)

Originally committed as revision 20961 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale