Disable FFmpeg hwaccel when APPIMAGE_BUILD is set

[ferdnyc]

As discussed in OpenShot/openshot-qt#4538, the hwaccel code in FFmpegReader/Writer should be completely disabled, when building for the Linux AppImages. This PR respects the CMake option 'APPIMAGE_BUILD', which we are already setting in the project server builds, and defines the symbol USE_HW_ACCEL=0 for the build. The #ifdefs around the hwaccel code will cause it to be omitted from the compiled classes entirely.

Fixes OpenShot/openshot-qt#4538

[codecov]

Codecov Report

Merging #779 (f51ba3f) into develop (c966918) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #779   +/-   ##
========================================
  Coverage    49.02%   49.02%           
========================================
  Files          183      183           
  Lines        15683    15683           
========================================
  Hits          7688     7688           
  Misses        7995     7995           

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[JacksonRG]

LGTM!