Support more aspect ratios (or custom aspect ratios)

[jportner]

Today I was trying to downscale a 4k (4096x1716, 2.39 aspect ratio) video to 1080p (1920x804), and I found that the options available are too limited:

I guess this AR is uncommon, but it would be great to add it in the dropdown.
Better yet, the ability to set custom width/height would be more future-proof and probably a bit easier to use, the current UI is a bit unwieldy.

[CineEncoder]

Hello! Ok, I'll add it to the next release.

[j0nik]

e.g. -vf scale=1920:-1 and vice versa would be great