Add a function to match a 2 element vector of uint16_t and use it in h263 and svq1

Add a function to match a 2 element vector of uint16_t and use it in h263 and svq1
instead of custom and bloated code to find an index into a w/h array.

Originally committed as revision 21113 to svn://svn.ffmpeg.org/ffmpeg/trunk