Fix touchpad pan gesture suppressed when editing masks

[da-phil]

A two-finger scroll (no CTRL) fell through to zoom whenever a mask overlay was visible, even with the pointer off the mask. gesture_pan now declines the pan only when the scroll would actually be consumed by the mask (pointer over a shape, or creating a size-adjustable shape) via the new dt_masks_scroll_over_mask() predicate. Elsewhere on the canvas, and while drawing a path (which has no scroll-adjustable parameter during creation), the scroll pans the image as expected.

Here is the updated behavior overview:

  ┌─────────────────────────────┬────────────────────┬──────────────────────────┐
  │           Gesture           │ Cursor inside mask │   Cursor outside mask    │
  ├─────────────────────────────┼────────────────────┼──────────────────────────┤
  │ Two-finger scroll (no Ctrl) │ resize mask        │ pan   (was zoom)         │
  ├─────────────────────────────┼────────────────────┼──────────────────────────┤
  │ Ctrl + two-finger scroll    │ opacity            │ zoom  (matches mouse)    │
  ├─────────────────────────────┼────────────────────┼──────────────────────────┤
  │ Pinch                       │ zoom               │ zoom                     │
  └─────────────────────────────┴────────────────────┴──────────────────────────┘

  ┌────────────────────────────────────────┬──────────────────────────────────────────┐
  │                 State                  │       Two-finger scroll (no Ctrl)        │
  ├────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Drawing a path                         │ pan (was zoom)                           │
  ├────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Creating circle/ellipse/gradient/brush │ adjust default size/border (unchanged)   │
  ├────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Pointer over an existing shape         │ resize                                   │
  ├────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Pointer off the mask                   │ pan                                      │
  └────────────────────────────────────────┴──────────────────────────────────────────┘

Disclaimer: this work has been co-created with Claude.

[da-phil]

@TurboGit I think this one qualifies for a 5.6.1, given that touchpad gestures were (properly) introduced in that release.

[ralfbrown]

Does this code correctly result in zooming while holding down the 'a' key even when the pointer is over a shape?

[da-phil]

Does this code correctly result in zooming while holding down the 'a' key even when the pointer is over a shape?

It does, with mouse wheel scroll gesture. On the touchpad the pointer is freezing as soon as "a" is pressed and all touchpad gestures ignored. It this how it should behave? I wasn't even aware of this alternative behavior using the "a" key.