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:611

Change-Id: Ie764c39a327d48b18d77246a028493b6270b9f88