This commit enables an adaptive prediction filter type selection
for sub8x8 block sizes. In speed 1, it re-uses the filter type of
collocated 8x8 block if it is tested in the rate-distortion optimization
loop, for the sub8x8 blocks. Otherwise, it runs the normal test
over all the three filter types. In speed 2, it re-uses the 8x8
block's prediction filter type, if available. Otherwise, force it
to be EIGHTTAP.