overview: Delay showing until there is gesture progress

overview: Delay showing until there is gesture progress

Currently swiping down in the session enters the overview, but
as there is no progress (clamped to 0 from negative), the only
visible effect is that the top bar transitions to transparent.

Avoid this glitch by delaying showing the overview until there
is some non-0 progress.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7456
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3777>
(cherry picked from commit fb30a94243c3ced887a8ccadb2987c27d61bf467)