vp9: postencode drop frame for screen content in CBR.

Encode the next frame at max q.

For layers: post_encode_drop is only check on base
spatial layer, and if base is post-encoded-dropped,
then whole superframe is dropped.

Added API to guard postencode dropping. Turned off by default.

Added unittest.

Change-Id: I42fee279014aca616f7a4d9b582cb2bf5da2f2e7
8 files changed