Fix: Window position creeps between executions on scaled monitors (#4443)

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* Closes <https://github.com/emilk/egui/issues/4442>
* Refactors active monitor detection so it can be called from multiple
locations.

Compare this gif to the one on the issue report.

![egui_window_position_no_creep](https://github.com/emilk/egui/assets/45777186/8e05d4fb-266e-48b9-9223-b65f16500a99)

### Investigation notes

- [`WindowSettings.inner_position_pixels` and
`WindowSettings.outer_position_pixels`](https://github.com/emilk/egui/blob/master/crates/egui-winit/src/window_settings.rs#L8-L12)
are stored in physical/pixel coordinates.
- `ViewportBuilder::with_position` expects to be passed a position in
_logical_ coordinates.
- Prior to this PR, the position was being passed from `WindowSettings`
to `with_position` [without any
scaling](https://github.com/emilk/egui/blob/master/crates/egui-winit/src/window_settings.rs#L61-L68).
This was the root cause of the issue.
- The fix is to first convert the position to logical coordinates,
respecting the scaling factor of the active monitor. This requires us to
first determine the active monitor, so I factored out some of the logic
in
[`clamp_pos_to_monitor`](https://github.com/emilk/egui/blob/master/crates/egui-winit/src/window_settings.rs#L130)
to find the active monitor.

crates/eframe/src/native/epi_integration.rs

@@ -38,7 +38,11 @@ pub fn viewport_builder<E>(
         }
         window_settings.clamp_position_to_monitors(egui_zoom_factor, event_loop);
 
-        viewport_builder = window_settings.initialize_viewport_builder(viewport_builder);
+        viewport_builder = window_settings.initialize_viewport_builder(
+            egui_zoom_factor,
+            event_loop,
+            viewport_builder,
+        );
         window_settings.inner_size_points()
     } else {
         if let Some(pos) = viewport_builder.position {