This means objects have an owner, where the chain eventually always
leads to a MetaContext. This also means that all objects can find their
way to other object instances via the chain, instead of scattered global
singletons.
This is a squashed commit originally containing the following:
cursor-tracker: Don't get backend from singleton
idle-manager: Don't get backend from singleton
input-device: Pass pointer to backend during construction
The backend is needed during construction to get the wacom database.
input-mapper: Pass backend when constructing
monitor: Don't get backend from singleton
monitor-manager: Get backend directly from monitor manager
remote: Get backend from manager class
For the remote desktop and screen cast implementations, replace getting
the backend from singletons with getting it via the manager classes.
launcher: Pass backend during construction
device-pool: Pass backend during construction
Instead of passing the (maybe null) launcher, pass the backend, and get
the launcher from there. That way we always have a way to some known
context from the device pool.
drm-buffer/gbm: Get backend via device pool
cursor-renderer: Get backend directly from renderer
input-device: Get backend getter
input-settings: Add backend construct property and getter
input-settings/x11: Don't get backend from singleton
renderer: Get backend from renderer itself
seat-impl: Add backend getter
seat/native: Get backend from instance struct
stage-impl: Get backend from stage impl itself
x11/xkb-a11y: Don't get backend from singleton
backend/x11/nested: Don't get Wayland compositor from singleton
crtc: Add backend property
Adding a link to the GPU isn't enough; the virtual CRTCs of virtual
monitors doesn't have one.
cursor-tracker: Don't get display from singleton
remote: Don't get display from singleton
seat: Don't get display from singleton
backend/x11: Don't get display from singleton
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2718>
c390f70edc