The encoder and decoder will now use the 16-bit (high-bitdepth)
coding path, regardless of the encoding profile.

This is a step towards the removal of the 8-bit (low-bitdepth)
coding path (which remains compile-checked at the moment).

BUG=aomedia:39

Change-Id: I6f8e8f1ce7a3bf1d3da7fda8461b1214ef579c64