Improving Support for Newer FFMPEG versions

[jonoomph]

• Fixing Audio Channel Layout detection for WAV (and other formats) that don't include a channel layout in the header (fallback for STEREO and MONO layouts based on # of channels)
• Fixed crashing Caption unit tests, and made them more resilient to pixel variations and font variations
• Removed a "flush encoders" call in FFmpegWriter, which was breaking exports on newer versions of FFMPEG and breaking some hardware acceleration

[codecov]

Codecov Report

Attention: Patch coverage is 5.26316% with 36 lines in your changes missing coverage. Please review.

Project coverage is 54.24%. Comparing base (044bb22) to head (5be0f4a).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/Caption.cpp	3.03%	32 Missing ⚠️
src/FFmpegReader.cpp	20.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #972      +/-   ##
===========================================
- Coverage    54.27%   54.24%   -0.03%     
===========================================
  Files          182      182              
  Lines        16506    16513       +7     
===========================================
  Hits          8958     8958              
- Misses        7548     7555       +7     

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