ui/gtk: use widget size for cursor motion event

The gd_motion_event size has some calculations for the cursor position,
which also take into account things like different size of the
framebuffer compared to the window size.
The use of window size makes things more difficult though, as at least
in the case of Wayland includes the size of ui elements like a menu bar
at the top of the window. This leads to a wrong position calculation by
a few pixels.
Fix it by using the size of the widget, which already returns the size
of the actual space to render the framebuffer.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Message-Id: <20230320160856.364319-1-ernunes@redhat.com>