In the native backend, the MetaRenderer manages the view by creating one
per CRTC, but until now the MetaStageX11 managed the view for the X11
backend. This caused some issues as it meant meta_renderer_get_views()
not returning anything, and that the view of the X11 screen not being a
MetaRendererView, while in the other backends, all views are.
Fix this by moving the view management responsibility to
MetaRendererX11Cm, and have MetaStageX11 only operate on it via
meta_renderer_x11_cm_*() API. The MetaRendererX11Cm takes care of making
sure the view is always added to the list in the renderer, and turning
X11 screen sizes into "layouts" etc.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1251
Split up the MetaRendererX11 class into one for when running as a
X11 compositing manager, and one for when running as a nested Wayland
compositor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
