Partial Crop/Zoom Filter

[jimmy]

The partializable zoom filter is great, thanks, I use it often.

But it seems to have two (no doubt intended) behaviors:
- adding black bars to maintain the aspect ratio, or
- adding or modifying top/bottom/left/right values to maintain aspect ratio.

What I would really like is the ability to crop exactly what is needed and then resize to source, even if it skews the aspect ratio. Using the zoom filter to
make small adjustments a slight skewing of the aspect ratio would be tolerable, but as it is now, it forces either the black bars or unneeded cropping.

[szlldm]

A no padding option will be available in the next nightly/release (whichever happens first)

[eumagga0x2a]

Definitely a nightly as we need to evaluate updating FFmpeg. n4.4.1 doesn't bring a lot of relevant fixes, but nevertheless I'd rather delay the release by two more weeks than go with outdated FFmpeg from the start on.

[jimmy]

Super. Thanks to you both for all you do.

[jimmy]

Latest nightly, r211112, thanks for adding the none/stretch option.