Move FFmpeg hwaccel checks to CMake, eliminate #pragma messages

[ferdnyc]

This PR renames the HAVE_HW_ACCEL preprocessor symbol to USE_HW_ACCEL, and adds a new CMake build option of the same name (default ON). This allows hardware acceleration to be disabled even when building with FFmpeg 3.4+ (by adding -DUSE_HW_ACCEL=0 to the CMake command line).

In addition, the #pragma messages about hardware acceleration are removed from FFmpeg{Reader,Writer}.cpp and replaced with a CMake FeatureSummary message that accounts for both the value of USE_HW_ACCEL, and whether the FFmpeg version in question supports acceleration.

Normal output (with FFmpeg 3.4+)

$ cmake -B build -S . [...]
-- Displaying feature summary

Build configuration:
-- The following features have been enabled:

 * FFmpeg hwaccel, GPU-accelerated routines (FFmpeg 3.4+)
 * OpenCV algorithms, Use OpenCV algorithms
 * Documentation, Build API documentation with 'make doc'
 * Unit tests, Compile unit tests for library functions
 * Testrunner, Run unit tests with 'make test'

-- The following OPTIONAL packages have been found:

 * ImageMagick
 * cppzmq
 * OpenCV (required version >= 4)
 * Threads
 * PythonInterp (required version >= 3)
 * PythonLibs (required version >= 3)
 * Ruby
 * Doxygen
 * PkgConfig

-- The following RECOMMENDED packages have been found:

 * UnitTest++
   Unit testing framework

-- The following REQUIRED packages have been found:

 * OpenShotAudio (required version >= 0.2.0)
 * jsoncpp
 * Qt5Widgets
 * FFmpeg
 * OpenMP
 * ZeroMQ
 * Protobuf (required version >= 3)
 * Qt5Core (required version >= 5.15.2)
 * Qt5Gui
 * Qt5
 * SWIG (required version >= 3.0)

-- The following features have been disabled:

 * Coverage, analyze test coverage and generate report
 * IWYU (include-what-you-use), Scan all source files with 'iwyu'

-- The following OPTIONAL packages have not been found:

 * Resvg

Manually disabled

$ cmake -B build -S . -DUSE_HW_ACCEL=0 [...]
-- Displaying feature summary

Build configuration:
-- The following features have been enabled:

 * OpenCV algorithms, Use OpenCV algorithms
 * Documentation, Build API documentation with 'make doc'
 * Unit tests, Compile unit tests for library functions
 * Testrunner, Run unit tests with 'make test'

-- The following OPTIONAL packages have been found:

 * ImageMagick
 * cppzmq
 * OpenCV (required version >= 4)
 * Threads
 * PythonInterp (required version >= 3)
 * PythonLibs (required version >= 3)
 * Ruby
 * Doxygen
 * PkgConfig

-- The following RECOMMENDED packages have been found:

 * UnitTest++
   Unit testing framework

-- The following REQUIRED packages have been found:

 * OpenShotAudio (required version >= 0.2.0)
 * jsoncpp
 * Qt5Widgets
 * FFmpeg
 * OpenMP
 * ZeroMQ
 * Protobuf (required version >= 3)
 * Qt5Core (required version >= 5.15.2)
 * Qt5Gui
 * Qt5
 * SWIG (required version >= 3.0)

-- The following features have been disabled:

 * Coverage, analyze test coverage and generate report
 * IWYU (include-what-you-use), Scan all source files with 'iwyu'
 * FFmpeg hwaccel, GPU-accelerated routines (FFmpeg 3.4+)

-- The following OPTIONAL packages have not been found:

 * Resvg

As a fallback, if USE_HW_ACCEL is not defined in the generated compile commands at all (which it will not be, if USE_HWACCEL is TRUE, but the FFmpeg version cannot be confirmed to support acceleration), then FFmpegUtilities.h will set USE_HW_ACCEL using the previous version-detection code.

[codecov]

Codecov Report

Merging #645 (e6d4ab3) into develop (691fc83) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #645   +/-   ##
========================================
  Coverage    51.53%   51.53%           
========================================
  Files          151      151           
  Lines        12231    12231           
========================================
  Hits          6303     6303           
  Misses        5928     5928           

Impacted Files	Coverage Δ
src/FFmpegReader.cpp	68.35% <ø> (ø)
src/FFmpegReader.h	66.66% <ø> (ø)
src/FFmpegUtilities.h	100.00% <ø> (ø)
src/FFmpegWriter.cpp	59.71% <ø> (ø)

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[github-actions]

Merge conflicts have been detected on this PR, please resolve.

[ferdnyc]

I don't see a down side to this, and hearing no objections...