Philippe Gorley authored Jan 30, 2019 and Adrien Béraud committed Mar 06, 2019

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