removed global vars so that multiple swscalers can be used

experimental upscaling mode (-sws 3)
general convolution filters support (unfinished)
bugfix for bicubic upscaling
assertion checking if defined MP_DEBUG
checking of the input/output size instead of segfault if its very large

Originally committed as revision 4277 to svn://svn.mplayerhq.hu/mplayer/trunk/postproc