test: vaapicontext: support wayland display

test: vaapicontext: support wayland display

On Wayland, The whole gtk window is one Wayland surface. So
gtk_widget_get_window() must be called on the top-level widget.
For any other widget the following gdk_window_ensure_native() may create a
new top-level Wayland surface that is never visible.

As a result, the coordinates passed to
gst_video_overlay_set_render_rectangle() must be relativ to the top-level
window. Otherwise the video is placed incorrectly.

Original-Patch-By: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/342>