libvpx

Add dynamic resize logic for 1 pass CBR.

Decision to scale down/up is based on buffer state and average QP
over previous time window. Limit the total amount of down-scaling
to be at most one scale down for now.

Reset certain quantities after resize (buffer level, cyclic refresh,
rate correction factor).

Feature is enable via the setting rc_resize_allowed = 1.

Change-Id: I9b1a53024e1e1e953fb8a1e1f75d21d160280dc7