Fix docs build failure by re-indenting with spaces instead of tabs.

Trailing whitespace fixes. http://pullrequest.opencv.org/buildbot/builders/precommit_docs/builds/5152/steps/whitespace/logs/stdio

mingw build fix.

Current implementation returns NaN for some configs of FFMPEG, call default get_fps() instead (as also used by PROP_POS_MSEC)

not only print file-not-found as a warning but also print the name of the problematic file and the uri used for the open attempt

Catched on Ubuntu 12.04 with FFMPEG:
  codec:                     YES (ver 53.35.0)
  format:                    YES (ver 53.21.1)
  util:                      YES (ver 51.22.3)
  swscale:                   YES (ver 2.1.0)
  resample:                  NO
  gentoo-style:              YES

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.

- fix compilation for old libraries
- update codec/tag selection logic
- add documentation note about MP4 container tags

Conflicts:
	modules/highgui/src/cap_ffmpeg_impl.hpp

Conflicts:
	modules/highgui/src/cap_ffmpeg_impl.hpp

FFmpeg now requires that frames allocated with avcodec_alloc_frame are
freed with avcodec_free_frame.

(cherry picked from commit 77578d41)

Conflicts:
	modules/highgui/src/cap_ffmpeg_impl.hpp

The 'sysctl' syscall has been strongly deprecated on Linux for ages.
Currently, on old architectures it will spam syslog whenever used, and on
newer ones it's missing from the headers altogether.  Opencv has migrated
away on Linux already, but #includes were left lingering.  This commit
removes them on non-__APPLE__, unbreaking x32 (and probably others).

Implement VideoCapture::get, CvCapture::getProperty, IVideoCapture::getProperty as constant methods.

avformat_find_stream_info was introduced in Libav avformat 53.3,
but it was only merged to FFmpeg in avformat 53.6.

In Libav avformat 53.3-53.5 av_find_stream_info is not removed
(only deprecated), so this shouldn't break building with that.

for libpng added.

VideoWriter memory leak solved.

Based on pull request #685

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

Add version checks around uses of AVCodecID and av_opt_set, since these aren't
defined for older versions of ffmpeg.