vp9-svc: Fix to frame dropping in svc mode.

Don't advance the svc frame counters on dropped frame,
since this can break the referencing scheme and lead
to a crash/assert.

Updated svc-datarate unittest to add a lower bitrate test.

Change only affects 1 pass cbr svc, with frame dropper enabled.

Change-Id: Ibb7530b7a587a9344d46898d9286fd9e2ef0779c
2 files changed