vp8: Fix to multi-res-encoder for skipping streams.

For the vp8 simulcast/multi-res-encoder:

Add flags to keep track of the disabling/skipping of
streams for the multi-res-encoder. And if the lower spatial
stream is skipped for a given stream, disable the motion
vector reuse for that stream.

Also remove the condition of forcing same frame type
across all streams.

This fix allows for the skipping/disabling of the base
or middle layer streams.

Change-Id: Idfa94b32b6d2256932f6602cde19579b8e50a8bd
3 files changed