`has()` only tests for an argument presence which is always true for
arguments with default values. Use `get<bool>()` to check the value
instead.

and not consider its value. treat arguments with a set default value as
present.

`A || !A` is `true` so write `(A && B) || !A` as `B || !A`

Migration have been done for these ffmpeg/libav changes:

2013-12-11 - 29c83d2 / b9fb59d,409a143 / 9431356,44967ab / d7b3ee9 - lavc 55.45.101 / 55.28.1 - avcodec.h
  av_frame_alloc(), av_frame_unref() and av_frame_free() now can and should be
  used instead of avcodec_alloc_frame(), avcodec_get_frame_defaults() and
  avcodec_free_frame() respectively. The latter three functions are deprecated.

2012-10-08 - ae77266 / 78071a1 - lavu 51.74.100 / 51.42.0 - pixfmt.h
  Rename PixelFormat to AVPixelFormat and all PIX_FMT_* to AV_PIX_FMT_*.
  To provide backwards compatibility, PixelFormat is now #defined as
  AVPixelFormat.
  Note that this can break user code that includes pixfmt.h and uses the
  'PixelFormat' identifier. Such code should either #undef PixelFormat
  or stop using the PixelFormat name.

Disables TLS copy constructor and operator, as they can lead to errors and reservation of too much keys in TLS storage;

gather method was added to TLS to gather data from all threads;

Reverted _doc.py to Python 2-only version. Interim measure to prevent docs from failing during build, until all samples are updated with Python 2/3 compatibility.