Adds a cmake variable (DEBUG_FFMPEG_MUX) which enables FFmpeg debug
output in the ffmpeg-mux subprocess, and if on Windows, shows the
console window of the ffmpeg-mux subprocess so the current output can be
seen.
Code submissions have continually suffered from formatting
inconsistencies that constantly have to be addressed. Using
clang-format simplifies this by making code formatting more consistent,
and allows automation of the code formatting so that maintainers can
focus more on the code itself instead of code formatting.
This adds functions for piping a command line program's stdin or stdout.
Note however that this is unidirectional only.
This will be especially useful later on when implementing MP4 output,
because MP4 output has to be piped to prevent unexpected program
termination from corrupting the file.
