waylandsink: create/destroy the display when entering/leaving the READY state instead...

waylandsink: create/destroy the display when entering/leaving the READY state instead of PAUSED

This is the only way to get the negotiation working with the dynamic
detection of formats from the display, because the pipeline needs
to know the supported formats in the READY state and the supported
formats can only be known if we open the display.

Unfortunately,in wayland we cannot have a separate connection to
the display from the rest of the application, so we need to ask for a
window handle when going to READY in order to get the display from it.

And since it's too early to create a top level window from the state
change to READY, create it in render() when there is no other window.

This also changes set_window_handle() to not support window handle
changes in PAUSED/PLAYING (because it's complex to handle and useless
in practice) and make sure that there is always a valid display pointer
around in the READY state.