Use _com_ptr_t instead of CComPtr in ComPtr wrapper to avoid ATL dependency.

The Mutex manager registered with ffmpeg must reset the mutex to NULL after destruction, otherwise ffmpeg will give the invalid mutex to the next mutex manager when it asks it to CREATE a new mutex.
See ffmpeg code: http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/utils.c;h=28c5785398fcf11a3d3c70a8cd09e9df798e2734;hb=HEAD#l3423

Cherry picked from head (file has moved but issue is the same).
Conflicts:
	modules/videoio/src/cap_ffmpeg_impl.hpp

Currently, FFMPEG source files are included in highgui project file regardless of CMake WITH_FFMPEG option.
After applying this PR FFMPEG files are included only if WITH_FFMPEG option is enabled.

Taken from 3f076552