waylandsink: get the external display handle using GstContext

waylandsink: get the external display handle using GstContext

This drops the ugly GstWaylandWindowHandle structure and is much
more elegant because we can now request the display separately
from the window handle. Therefore the window handle can be requested
in render(), i.e. when it is really needed and we can still open
the correct display for getting caps and creating the pool earlier.

This change also separates setting the wl_surface from setting its size.
Applications should do that by calling two functions in sequence:

  gst_video_overlay_set_window_handle (overlay, surface);
  gst_wayland_video_set_surface_size (overlay, w, h);