workspacesView: Avoid setting invalid geometries on views

workspacesView: Avoid setting invalid geometries on views

The fullGeometry and the actualGeometry of the WorkspacesDisplay are set
from the allocation of the overviews ControlsManager and the
WorkspacesDisplay, that means they're only valid after those actors got
their allocations during Clutters allocation cycle.

Since WorkspacesDisplay._updateWorkspacesViews() is already called while
showing/mapping the WorkspacesDisplay, that allocation cycle didn't
happen yet and we end up either setting the geometries of the views to
null (in case of the fullGeometry) or to something wrong (a 0-sized
allocation in case of the actualGeometry).

So avoid setting invalid geometries on the views by initializing both
the fullGeometry and the actualGeometry to null, and then only updating
the geometries of the views after they're set to a correct value.

Note that this means we won't correctly animate the overview the first
time we open it since the animation depends on the geometries being set,
but is being started from show(), which means no allocations have
happened yet. In practice this introduces no regression though since
before this change we simply used incorrect geometries (see the 0-sized
allocation mentioned above) on the initial opening and the animation
didn't work either.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1119