huffyuv: fate: add a BGR24 test

It was previously untested.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
huffyuvdec: use unsafe bitstream reader

The reader reads in chunks of 11 bits at most, and at most 3 times. The unsafe
reader therefore may read 6 chunks instead of 1 in worst case, ie 8 bytes,
which is within the padding tolerance.

The reader ends up being ~10% faster. Cumulative effect of unsafe reading and
code block swapping on 3 sequences is for 1 thread, decoding time goes from
23.3s to 19.0s.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>