vp9-RC: Add drop_frame support to external RC

vp9-RC: Add drop_frame support to external RC

Supports single layer and svc. For svc only the
framedrop_mode = FULL_SUPERFRAME_DROP is allowed
for now.

Dropping frames due to overshoot is enabled by the
oxcf->drop_frames_water_mark, which is zero as default.
Note that this CL also allows for drop/skip encoding of
enhancement layers if that layer bitrate is zero.

max_consec_drop is also added, set to INT_MAX as default.
Note that max_consec_drop is only used for svc mode.
It has not been added yet for single layer in libvpx encoder.

Tests added for single layer and svc case.

Change-Id: Ic12f6a0eb3fbf07d8eb8456c46cec27b2e1930d3