Adds support for one-pass rc-enabled SVC encoder with callbacks for
getting per-layer packets.
- the callback function registration is implemented as an encoder
control function.
- if the callback function is not registered, the old way of
aggregating packets with superframe will take effect.
- one more control function “VP9E_GET_SVC_LAYER_ID” has been
implemented to get the temporal/spatial id from the encoder
within the callback. This can be used to get the ids to put on RTP
packet.

Change-Id: I1a90e00135dde65da128b758e6c00b57299a111a