accel: add hardware encoding

Adds VAAPI and VideoToolbox hardware encoders.

Abstracts hardware related field accesses from the encoder and decoder
to put them in HardwareAccel. They are freed in ~HardwareAccel.

Limits bitrate when hardware encoding h264, else it can easily go up to
a few MiB/s.

Change-Id: I7d847d8ab3e4c9692341f038ce6d5dd76562e606