libs: wayland: update the opaque region in set_render_rect

libs: wayland: update the opaque region in set_render_rect

gst_vaapi_window_wayland_set_render_rect() may be called from an arbitrary
thread. That thread may be responsible for making the window visible.

At that point another thread will block in gst_vaapi_window_wayland_sync()
because the frame callback will not be called until the window is visible.

If that happens, then acquiring the display lock in
gst_vaapi_window_wayland_set_render_rect() would result in a deadlock.

Cache the size of the opaque rectangle separately and create the opaque
region right before applying it to the surface.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/342>