ClutterLayerNode is a private paint node that paints the
contents of its subnodes into an offscreen buffer, then
paints this entire offscreen buffer at the previous draw
framebuffer with the given opacity.
It is specially interesting to the future ClutterContent
implementation of MetaShapedTexture, for the NVidia and
rotation cases where the contents are painted on an
offscreen.
Expose ClutterLayerNode as public API.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/405
41ece851d6