libvpx

High-level hooks for Profile 2 (10/12 bit)

Adds some high-level hooks for profile 2 before further
progress on the implementation.

According to the definitiion in this patch:
1. Profile 2 only supports 10 or 12 bit color but not 8
2. Profile 2 supports all color sampling modes: 444, 422 and 420,
and alpha plane.
3. Profile 3 is currently undefined.

Please consider the definition carefully and suggest modifications
to the definition as needed.

Change-Id: I5b284fc679e54ac5aee171af72fa7994cfd28995