Stream output file to stdout

[Meakk]

Implement #1101

[Meakk]

@mwestphal @snoyer please take a look :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.45%. Comparing base (5055e64) to head (7250664).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1326   +/-   ##
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  Coverage   96.44%   96.45%           
=======================================
  Files         146      146           
  Lines        8746     8765   +19     
=======================================
+ Hits         8435     8454   +19     
  Misses        311      311           

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[snoyer]

The key point here is that we want to ensure nothing has been written to stdout before we use it to output render data, otherwise we will quietly output a corrupted PNG.

Setting the log level to ERROR early seemed to do a good enough job when I suggested it but I haven't fully tested it.
Possible issues include:

• if ERROR level messages go to stdout and can we have non-critical error messages (ie. not exceptions and we still proceed to render)
• if there's any code that output to stdout directly (most likely external code not going through f3d::log)
• if there are cases where you'd want to render to stdout and still see log messages somehow

Maybe it'd be better to not mess with the verbosity levels and instead redirect stdout to stderr from the start of the app if we know we'll be rendering to stdout?
That way any logging would still get to the terminal through stderr and only the PNG data would go to stdout (f3d foo.bar --verbose --output=- > /tmp/f3d.png would output both the full log and a valid PNG).
Would that be just a dirty hack? Are there any critical implications I'm not seeing?

[github-actions]

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