Encoding at different FPS to camera / variable framerate encoding

[kodonnell]

I can't quite run my computer vision pipeline and record video at full FPS. But I edited Encoder.encode to only encode every nth frame (n=FPS gives 1 FPS video), and now it's happy. This is fine for my use case (as the video is for verification purposes only), and I don't suspect it'll be too valuable to others unless they're fine-tuning performance like me (at which point they can just overwrite the class like me) ... but maybe it'll be useful in the standard lib (either how I did, or wherever Picamera2 calls the encode function), especially as it's pretty trivial.

Fun thought ... it could allow for variable framerate encoding! Maybe that's best as an example e.g. when there's motion happening, encode at full FPS, otherwise drop down to 1FPS etc. That's actually potentially really useful ... but again, maybe only to a niche group of users.

What do you reckon?