wayland: Use the pointer enter serial when setting cursor surface

wayland: Use the pointer enter serial when setting cursor surface

wl_pointer.set_cursor is rejected if the serial number doesn't match
the enter serial number for the wl_pointer.  We passed the right serial
number when setting the cursor surface in response to the enter event.
Later set_cursor requests fail, but we can still attach new buffers to
our cursor surface, which is why the cursor changed, but the hotspot
didn't update.  Clicking in the decoration results in a leave/enter pair
which triggers wl_pointer.set_cursor with the right serial. That's why
clicking the decoration sets the right cursor.

https://bugzilla.gnome.org/show_bug.cgi?id=695512