Cursor hotspot and cursor image are not updated atomically on Wayland

Qt updates the cursor in two steps:

• set hotspot
• attach a buffer

This can result in the cursor unexpectedly jumping if the compositor updates the cursor between set hotspot and attach a buffer steps, e.g. https://bugs.kde.org/show_bug.cgi?id=488398. In order to ensure that cursor updates are atomic and there are no glitches, QtWayland needs to update the cursor differently:

• if the cursor needs to be updated after receiving an enter event, a new buffer should be attached to the surface before the wl_pointer.set_cursor() request

wl_surface.offset(surface, 0, 0)
wl_surface.attach(surface, buffer, 0, 0)
wl_surface.commit(surface)
wl_pointer.set_cursor(pointer, surface, hot_x, hot_y)

• if the cursor needs to be updated after wl_pointer.set_cursor(), wl_surface.offset should be used

  wl_surface.offset(surface, old_hot_x - new_hot_x, old_hot_y - new_hot_y)
  wl_surface.attach(surface)
  wl_surface.commit(surface)

Note that wl_surface.offset doesn't take an absolute hotspot position, instead it takes an offset relative to the old hotspot position. wl_pointer.set_cursor takes an absolute hotspot position.