waylandsink: remove the buffer from the surface when going PAUSED -> READY

waylandsink: remove the buffer from the surface when going PAUSED -> READY

This essentially hides the video and allows the application to
potentially draw a black background or whatever else it wants.
This allows to differentiate the "paused" and "stopped" modes
from the user's point of view.

Also reworded a comment there to make my thinking more clear,
since the "reason for keeping the display around" is not really
the exposed() calls, as there is no buffer shown in READY/NULL
anymore.