vp9-svc: Allow for setting frame drop thresholds per layer.

vp9-svc: Allow for setting frame drop thresholds per layer.

Add encoder control to set the frame drop thresholds per
spatial layer, and add a frame drop mode: 0 = per-layer drop,
and 1 = constrained drop mode (a drop on a given layer forces
drops to all upper layers).

Default is mode 0 (per-layer dropping).
Implementation for mode 1 will come in subsequent change.

If the control is not used, then the spatial layer frame
drop thresholds (water mark) are all equal and set to the value
given by the encoder config (oxcf->drop_frames_water_mark).

Bump up the ABI version.

Change-Id: Id038d4181b86fa98b3d44d026f96d5f344d81629