image: Rotate around gesture center (sometimes)
We are currently always rotating around the center of the image. Not even around the center of the viewport.
This might currently be non-trivial to implement because the adjustments/scrolled window might not be able to do all the range of movement we need.
Touchscreen
We probably always want to rotate around the gesture center so the image pixels follow the fingers. As should already be the case for zoom and pan.
Touchpad
Maybe always around the center of the viewport?
Button/Menu/Shortcut
Maybe also around the center of the viewport?