Add FPS field if available

[kiddkaffeine]

I believe the logic here should be correct, but I am light on samples to test with.

ffmpeg does not add this info by default, but permits you to manually specify these values, e.g.:

ffmpeg -I input.m4v -c:v libaom-av1 -bsf:v av1_metadata=tick_rate=30000/1001:num_ticks_per_picture=1 -an output.obu

[flibitijibibo]

Not a maintainer but this seems reasonable - it may be worth documenting that this is likely not going to be available unless the AV1 exporter specifies this, but there aren't docs in the headers so I'm not sure what the best fix is without writing a bunch of docs first.

[flibitijibibo]

We ended up talking about this in the FNA Discord and decided to separate this into bool df_guessVideoFramerate, since the other values are guaranteed and we want this to stick out as possible, but not guaranteed. This also allows us to document the one function with the ffmpeg parameters so that people know how to use the function correctly.

[kiddkaffeine]

Updated per discussion.

If the info is not available, the data at the fps pointer is left alone (undefined). Is that the correct desired behavior?

[flibitijibibo]

This looks right - with the documentation this should be ready.