wayland: Reset pending offset in GL path
The GL Wayland drag surface code path has a bug where it does not reset the hotspot, so if a GL-backed draw surface redraws without resizing or resetting the hotspot, it moves away. This MR adds a test and fixes the bug. The cairo path and the X11 backend are not affected.
Before:
Screencast_from_2023-03-15_16-17-25
After:
Screencast_from_2023-03-15_16-19-21
cc @otte @matthiasc