libvpx

Relax maximum Q for extreme overshoot.

Added code to relax the active maximum Q in response
to extreme local overshoot to reduce bandwidth peaks.

The impact is small in metrics terms, but it this helps reduce
bandwidth spikes and overall overshoot in a number of
clips in our tests sets (especially the YT test set).

In particular this should help prevent very big spikes where a clip
is mainly easy but has a short hard section. In such a case a choice
of maximum Q for the clip as a whole may allow us to hit the overall
target rate but give some extreme spikes. The chunked encoding in YT
mitigates this problem but it can show up where a longer clip is
coded as a single chunk.

Change-Id: I213d09950ccb8489d10adf00fda1e53235b39203