libvpx

Make encoder buffer allocation dynamic

Frame buffers are now allocated dynamically on-demand.

Entries in the reference frame map, cm->ref_frame_map,
may now be set to -1 (INVALID_IDX) to indicate that
there is not a valid reference buffer in that "slot".

All slots in the reference frame map are now initialized
to the empty state (-1) and each buffer is initialized
to have a reference count of 0.

Change-Id: Id1afe98de98db4ae8b2dfefed7889c3b28c68582