Allow custom aspect ratio scaling "zoom" (useful for ultrawide + forced black bars)

[ckcr4lyf]

Sometimes even 21:9 (or 18:9) movies have forced black bars (e.g. BluRay remuxed at 16:9). One a normal display it is fine, but on an ultrawide display, this results in the actual content not being able to take full advantage of the ultrawide resolution.

This can be addressed by letting the user decide how much they want to scale the video at a more custom level.

I've implemented a PR for this, which has an example of how its used, let me know what you think: #7627