flag for diagonal interpolation

the primary reason for this change is that previously MC up to 1/4 pel
matched H.264 exactly and that increases the risk of stumbling over
patents

secondly this allows 0.10 db or more quality gain by choosing a longer
filter and the filter could also be chosen optimally for each frame
though that of course would cause speed loss at the decoder and encoder
side ...

Originally committed as revision 10436 to svn://svn.ffmpeg.org/ffmpeg/trunk