Add preset to VideoEncoder API

[Dan-Flores]

This PR adds preset as an option in the VideoEncoder API.
To review just these changes, see 2a49a3d.

Preset is a readable string in libx264, so the docstring suggests those strings. Occasionally (as in the case of the libsvtav1 codec), preset can be an int, so both types are accepted.

To test, test_video_encoder_against_ffmpeg_cli is updated to try combinations of pixel_format, crf, and preset.