vp9-svc: Frame dropper for SVC.

vp9-svc: Frame dropper for SVC.

If a given spatial layer decides to drop, due to the
buffer/overshoot conditions for that layer, then drop
that current spatial layer and all spatial layers above.

In the current implementation the svc frame counter
(and hence the pattern for the non-flexible SVC case)
are updated on frame drops.

Also add last spatial layer encoded to the pkt.
This is useful for RTC applications that enable
frame dropping for SVC.

Update to the SVC datarate tests:
enabled frame dropper on all SVC datarate tests, and
made a fix to properly set the temporal_layer_id, which
works now even on frame drops.

Change-Id: If828c193f3cb6b1839803fd52fe9fbbda5b5a039