libvpx

Fixes to buffer update for temporal layers.

When a frame is dropped due to |buffer_level| < 0 for a given temporal layer,
the buffer level for the upper temporal layers was not updated (in calc_pframe_target_size()).
This change fixes that.

Also, use the layer per-frame-bandwidth for updating the buffer level
of the higher layers when a frame is dropped.

Change-Id: I660c23f3229b47e9d124a950b480314b4307c5a8