Enhance FindFFmpeg.cmake with fallback version parsing, FFmpeg_VERSION, etc.

[ferdnyc]

My recent PR #693 depends on the availability of the avformat_VERSION value in CMake, just as a previous PR added a dependency on the avcodec_VERSION for determining hardware-acceleration support. Previously, FFmpeg component versions were only available if PkgConfig had been used to retrieve the FFmpeg configuration.

This PR enhances the Find module so that, if PkgConfig is not available/used, the component versions are parsed out of the relevant version.h file in each component's include directory.

Along the way, it also makes several other enhancements to the Find module:

• Probing of non-requested FFmpeg components (a serious time-waster) is eliminated, so if the find_package() line specifies COMPONENTS or OPTIONAL_COMPONENTS only those libraries will be available.
• All component variables now use the CMake standard naming of FFmpeg_<component>_<var>, instead of the previous <component>_<var>.
• find_package_handle_standard_args() is now called with HANDLE_COMPONENTS to verify the component list against FFmpeg_FIND_COMPONENTS
• The overall FFmpeg_VERSION value is now parsed from libavutil/ffversion.h and published as the module's VERSION_VAR

[codecov]

Codecov Report

Merging #694 (e2b685d) into develop (09eb807) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #694   +/-   ##
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  Coverage    50.41%   50.41%           
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  Files          155      155           
  Lines        13315    13315           
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  Hits          6713     6713           
  Misses        6602     6602           

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[ferdnyc]

The version number changes particularly affect the WIN32 platform, which previously lacked FFmpeg component versions because PkgConfig was not used to discover the library.

[ferdnyc]

• Probing of non-requested FFmpeg components (a serious time-waster) is eliminated, so if the find_package() line specifies COMPONENTS or OPTIONAL_COMPONENTS only those libraries will be available.

(Except for avutil, which will always be resolved due to the aforementioned parsing of libavutil/ffversion.h.)