waylandsink: create and maintain the subsurface inside the sink

waylandsink: create and maintain the subsurface inside the sink

This means that the given surface in set_window_handle can now be
the window's top-level surface on top of which waylandsink creates
its own subsurface for rendering the video.

This has many advantages:
* We can maintain aspect ratio by overlaying the subsurface in
  the center of the given area and fill the parent surface's area
  black in case we need to draw borders (instead of adding another
  subsurface inside the subsurface given from the application,
  so, less subsurfaces)
* We can more easily support toolkits without subsurfaces (see gtk)
* We can get properly use gst_video_overlay_set_render_rectangle
  as our api to set the video area size from the application and
  therefore remove gst_wayland_video_set_surface_size.