ecore_wl2: Add API ecore_wl2_window_surface_flush

ecore_wl2: Add API ecore_wl2_window_surface_flush

This allows something that only has the Ecore_Wl2_Window (ie: something
that isn't engine code) to force dropping of all the buffers.

This should be safe to call at any time as the buffer handling logic
will properly cleanup the buffers when async render is done with them
or the compositor releases them.

This will eventually be used when a wayland client receives a
wl_output.leave events to indicate it isn't displayed on any outputs.