Select scan tables for 8x4 and 4x8 blocks only once.

Patch by Christophe GISQUET ([christophe][gisquet] <whirlpool sign> free <spot> fr)
Thread [PATCH] Determine once and for all zz scan tables to use in VC-1 VLD

Originally committed as revision 11525 to svn://svn.ffmpeg.org/ffmpeg/trunk