Based on pull request #685

Set ptr to NULL, so this method can be called repeatedly. This fixes a crash after unplugging web cam and trying to re-scan the cameras.

This also fixes few clang build errors

All tests passed!

TIFF build was fixed;
Direct Show back-end was enabled;
More accurate guards implemented.

GPU module turned off;
TIFF support turned off;
Windows Managment and Video IO (VFW) turned off.

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

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

Fixed issues: #2737 #2848

- added webp (lossy and lossless without alpha channel)
- added tests

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

Add a call to avcodec_get_context_defaults3, this sets per-codec default values
and in the case of libx264 will stop VideoWriter::open (with fourcc=x264) from
failing due to libx264 complaining about broken ffmpeg default settings.

Set some additional libx264 encoder parameters.