Support for `filter_script:v`

[pomterre]

I am trying to integrate filter_script:v from the ffmpeg cli tool, useful for long, complex filter graphs where it gets limited when using in command line directly, and throws an too many args error. This is fixed by running from the file script instead.

Here is the ffmpeg command I am trying to replicate using ffmpeg-python:

ffmpeg -i video.mp4 -filter_script:v ../crop_filter.txt -c:v h264_nvenc -preset slow -crf 18 output.mp4

I could not find a way to use filter_script:v directly in ffmpeg-python. The current API allows for direct filter strings via .filter(), but not for external filter scripts.