Wayland clients are in control of their window size so the existing
mutter plugin API, which assumes size changes are synchronous, doesn't
work for them since when our size-change handler runs the MetaWindow's
size isn't final yet.
To fix this, the mutter plugin API was extended with a size-changed
vfunc that lets us know when the MetaWindow size has actually
changed. This way we can make the window snapshot and get the old
window size on the existing size-change handler and later, on the new
size-changed handler, get the new size and start the animation.
https://bugzilla.gnome.org/show_bug.cgi?id=770345
7bc1d57ad7