accel: skip main memory when hardware reencoding

If the video input is being decoded on the hardware, use the hardware
frames directly when encoding. Skips the transfer back to software in
the video sender and the transfer to hardware in the encoder.

Falls back to using the main memory as a middle man if the link between
encoder and decoder fails.

Change-Id: I59850e95e6952df71a461aad8e7dddda65f05ffb