retouch: opacity slider not updated when using mouse wheel on a shape
Steps to reproduce:
- Open retouch
- Draw a shape (eg. circle / heal)
- Click on the shape to select it: a slider "shape selected\nmask opacity" appears.
- Use control-wheel-down to reduce the shape's opacity
- The "shape selected\nmask opacity" slider moves
- The slider stays at 100%.
Note that de-selecting and re-selecting the shape does move the slider, so the data behind the slider and the mouse wheel are the same, it's just a matter of redrawing and/or updating the slider.