Move the v{Y,C}CoeffsBank vectors into the SwsContext, filling them in just

once when the scaler is initialized, instead of building them and freeing
them over and over. This gives massive performance improvements.
patch by Alan Curry, pacman*at*TheWorld*dot*com

Originally committed as revision 17589 to svn://svn.mplayerhq.hu/mplayer/trunk/postproc