Always cancel dragging when mouse is not down

To fix an upstream issue with mouse-up events not being propagated in Wayland with pen tablets during dragging, we cancel dragging and warn.

This allows us to continue to see the problem, but doesn't cascade into a user experience issue as dragging objects become unreliable.

Note: This might not need forward porting, the event system may be fixed in Gtk4.

Fixes #4830 (closed)