`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;

Fix a bug. When reading from a saved model, function decomposeCovs() will be called. And if covMatType is COV_MAT_DIAGONAL, covsEigenValues is computed using SVD and eigen values are sorted so that the order of eigen values is not preserved. This would lead to different result when calling function predict2. This issues is discussed here: http://stackoverflow.com/questions/23485982/got-different-empredict-results-after-emread-saved-model-in-opencv.

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.