Smarter VP8 prefetching
Prefetch all refs (including altref), but only if they've been used so far this
~2.5% faster overall.
TODO: Do something even smarter, like using how often each ref has been used
so far, so that a couple blocks of a rarely-used ref don't force us to prefetch
Originally committed as revision 24444 to svn://svn.ffmpeg.org/ffmpeg/trunk