libavcodec: Add an OpenH264 encoder wrapper

libavcodec: Add an OpenH264 encoder wrapper

Compared to existing, common opensource H264 encoders, this can be
useful since it has got a different license (BSD instead of GPL).

Performance- and qualitywise it is comparable to x264 in ultrafast
mode.

Hooking it up as an encoder in libavcodec also simplifies comparing
it against other common encoders.

This requires OpenH264 1.3 or newer. Since the OpenH264 API and ABI
changes frequently, only releases are supported.

To take advantage of the OpenH264 patent offer, the OpenH264 library
must not be redistributed, but downloaded at runtime at the end-user's
system.

Signed-off-by: Martin Storsjö <martin@martin.st>