Switch over to pyav

[tfaehse]

pyav provides lower-level python bindings for ffmpeg. This should in theory allow for better performance when reading and writing frames, and it could massively clean up the blurring speed. Potential upsides include:

• Not having to copy over the audio stream after the fact anymore, pyav supports this in one, clean write operation
• Improved performance
• More direct control over ffmpeg options

[tfaehse]

Current progress:

• Custom video readers and writers with context managers, work as drop-in replacement for imageio
• Performance is significantly better, partly because of multithreaded reading/writing of frames
• Copying audio data somewhat works - not for all input files

Audio copying/remuxing/resampling.. needs to be fixed before this can be pushed to main.

[tfaehse]

...imageio--ffmpeg is faster, and easier to handle.