CancellableThrough() throws if token is already cancelled

[gbrzeczek]

I've added the solution marked with 1. discussed in #468.

However, I think this doesn't solve the situation when cancellation is requested between calls to CancellableThrough() and ProcessAsynchronously() - I'd have to store the cancellation state somehow in the Argument Processor (maybe using a simple flag?). Please let me know what's the preferred solution.

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (f8407bc) 74.31% compared to head (4644007) 70.90%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
- Coverage   74.31%   70.90%   -3.42%     
==========================================
  Files         113      115       +2     
  Lines        2854     2911      +57     
  Branches        0      281     +281     
==========================================
- Hits         2121     2064      -57     
- Misses        733      739       +6     
- Partials        0      108     +108     

Files	Coverage Δ
FFMpegCore/FFMpeg/FFMpegArgumentProcessor.cs	84.97% <100.00%> (-3.41%)	⬇️
FFMpegCore/FFMpeg/FFMpegArguments.cs	83.56% <100.00%> (+2.91%)	⬆️
FFMpegCore/FFMpeg/FFMpegMultiOutputOptions.cs	84.61% <84.61%> (ø)
FFMpegCore/FFProbe/FFProbeAnalysis.cs	73.84% <0.00%> (ø)
FFMpegCore/FFMpeg/Arguments/OutputTeeArgument.cs	65.62% <65.62%> (ø)

... and 43 files with indirect coverage changes

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