Add custom pivot option (for scale & rotation)

[Robert-K]

I believe this effect would greatly benefit from an extra option: The pivot/transformation origin, relative to the input.

Most of the time, I want my transformations (rotation & scale) to be around the center of the frame. Currently, this requires me to tediously calculate & fine-tune the X/Y Offset.

My proposition would look something like this:

Pivot X: 0 --------- | --------- 1 Pivot Y: 0 --------- | --------- 1

Where a pivot (0.5, 0.5) would rotate and scale around the center of the input and (0,0) would result in the current behaviour.