vp8: Drop due to overshoot for non-screen content.

vp8: Drop due to overshoot for non-screen content.

For 1 pass CBR mode:
Apply the logic for dropping (and re-adjusting rate control)
due to large overshoot to the case of non-screen content when
drop_frames_allowed is enabled.

For the non-screen content case: add additional condition that
rate correction factor is close to minimum state, and flag to
constrain the frequency of the dropping.

Also handle the case of temporal layers and multi-res encoding.
Add some flags/counters to the layer context for temporal layers.
For multi-res: drop due to overshoot is checked on lowest stream,
and if overshoot is detected we force drops on all upper streams
for that frame.

This feature is to avoid large frame sizes on big content
changes following low content period.

No change in behavior for screen_content_mode = 2.

Change-Id: I797ab236cbbf3b15cad439e9a227fbebced632e6