Flatten the subsurface actor tree, making all surface actors children
of the window actor.
Save the subsurface state in a GNode tree in MetaWaylandSurface, where
each surface holds two nodes, one branch, which can be the tree root
or be attached to a parent surfaces branch, and a leaf, which is
used to save the position relative to child branch nodes.
Each time a surface is added or reordered in the tree, unparent all
surface actors from the window actor, traverse all leaves of the
tree and readd the corresponding surface actors back to the window
actor.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/664
Those are stub specialized classes for MetaWindowActor. This will
help ensuring that we do not execute X11-specific code paths on
pure Wayland clients.
The relationship between the window actor and the surface is the
following:
* Wayland: MetaWindowActorWayland + MetaSurfaceActorWayland
* X11: MetaWindowActorX11 + MetaSurfaceActorX11
* Xwayland: MetaWindowActorX11 + MetaSurfaceActorWayland
It is not possible to have MetaWindowActorWayland backed by a
MetaSurfaceActorX11 surface.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/368
