libvpx

Allocate buffers based on correct chroma format

The encoder currently allocates frame buffers before
it establishes what the chroma sub-sampling factor is,
always allocating based on the 4:4:4 format.

This patch detects the chroma format as early as
possible allowing the encoder to allocate buffers of
the correct size.

Future patches will change the encoder to allocate
frame buffers on demand to further reduce the memory
profile of the encoder and rationalize the buffer
management in the encoder and decoder.

Change-Id: Ifd41dd96e67d0011719ba40fada0bae74f3a0d57