waylandsink: remove the manual synchronization from pause/resume_rendering and use...

waylandsink: remove the manual synchronization from pause/resume_rendering and use subsurface sync/desync

Previously, in order to change the surface size we had to let the pipeline
redraw it, which at first also involved re-negotiating caps, etc, so a
synchronization with the pipeline was absolutely necessary.

At the moment, we are using wl_viewport, which separates the surface size
from the buffer size and it also allows us to commit a surface resize without
attaching a new buffer, so it is enough to just do:

gst_wayland_video_pause_rendering():
	wl_subsurface_set_sync()
gst_video_overlay_set_render_rectangle():
	wl_subsurface_set_position()
	wl_viewport_set_destination()
	wl_surface_damage()
	wl_surface_commit()
... commit the parent surface ...
gst_wayland_video_resume_rendering():
	wl_subsurface_set_desync()

This is enough to synchronize a surface resize and the pipeline can continue
drawing independently. Now of course, the names pause/resume_rendering are
bad. I will rename them in another commit.