gdkwindow-wayland: Obey Wayland buffer semantics

gdkwindow-wayland: Obey Wayland buffer semantics

We can't destroy buffers if they're in-use by the compositor. Well,
technically we can, but that is considered undefined by Wayland and
mutter won't cope with it very well -- it simply kills the client.

To solve this, we need to delay the destroy operation until the
compositor tells us that it's released the buffer. To do this, hold
an extra ref on the cairo surface as long as the surface is in-use
by the compositor.