Assert no 8x4/4x8 partition for scaled references

This commit adds a new configure option:

--enable-better-hw-compatibility

The purpose of the configure option is to provide information on known
hardware decoder implementation bugs, so encoder implementers may
choose to implement their encoders in a way to avoid triggering these
decoder bugs.

The WebM team were made aware of that a number of hardware decoders
have trouble in handling the combination of scaled frame reference
frame and 8x4 or 4x8 partitions. This commit added asserts to vp9
decoder, so when built with above configure option, the decoder can
assert if an input bitstream triggers such decoder bug.

Change-Id: I386204cfa80ed16b50ebde57f886121ed76200bf