the +-1 issue is limited to >2tap vertical filters, so bilinear upscale was unaffected
the new code is sometime faster sometimes slower but the difference is significant (~20%) so its optional and enabled with arnd=1
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@19177 b3059339-0415-0410-9bf9-f77b7e298cf2