fix: a11y: magnifier: correct Centered/OnEdge focal-point math and refresh focal point at zoom-in

- I used AI to assist in research of the focal point computation formulae for OnEdge and Centered modes.
- I used the formulas found in [KDE's zoom magnifier](https://invent.kde.org/plasma/kwin/-/blob/master/src/plugins/zoom/zoom.cpp) and converted them to Rust and the COSMIC focal point system.

- Remove unnecessary focal-point precomputation in `OutputZoomState::new`, because it is now updated on first zoom in anyway.

- Do focal point computation in Local coordinates in update_focal_point to eliminate local-global conversion confusion/ issues.

- ZoomMovement::OnEdge - single code path rewrite - old two-branch logic caused jumping and unpredictability at high zoom. Fixes [#2683](https://github.com/pop-os/cosmic-epoch/issues/2683), partially addresses [#2688](https://github.com/pop-os/cosmic-epoch/issues/2688).

- ZoomMovement::Centered - translation/clamp/invert - prevents "blow up" of calculation at zoom level > 2.0 that caused cursor to disappear off bottom right of screen. Fixes [#1503](https://github.com/pop-os/cosmic-comp/issues/1503).

- `is_level_change` - sets `previous_point` so that `animating_focal_point` interpolates instead of jumping. Still room for improvement (needs focal point recomputation per-frame during animation to prevent "boomerang" feel), but this is still an improvement over jumping.

- In `trigger_zoom`, call `update_focal_point` when `level > 1.0` - overlaps proposal in [#1859](https://github.com/pop-os/cosmic-comp/pull/1859) + the level gate to prevent stale focal point on zoom in. Fixes [#2511](https://github.com/pop-os/cosmic-comp/issues/2511) and [#1883](https://github.com/pop-os/cosmic-epoch/issues/1883). Partially addresses [#1420](https://github.com/pop-os/cosmic-comp/issues/1420) (item 1 only).

src/shell/mod.rs

@@ -2490,6 +2490,14 @@ impl Shell {
             increment: zoom_config.increment,
             movement: zoom_config.view_moves,
         });
+
+        if level > 1.0 {
+            self.update_focal_point(
+                seat,
+                seat.get_pointer().unwrap().current_location().as_global(),
+                zoom_config.view_moves,
+            );
+        }
     }
 
     pub fn update_focal_point(