Scene: Set transform layer before calling user content - #5884

This changes the `Scene` behaviour to call `set_transform_layer` prior
to calling the user content fn, rather than after.

### Motivation

This provides a simple way for the user to access the `TSTransform` that
will be applied to the `Scene` within the user content function, e.g.

```rust
ui.ctx().layer_transform_to_global(ui.layer_id())
```

Previously getting the transform like this still kind of worked, but
resulted in the user content lagging behind the actual scene position by
a single frame, which looks a bit strange.

With this PR, the user content using the transform no longer lags by a
frame, and matches the scene's transform perfectly.

Accessing the `TSTransform` of the `Scene` within the user content
function is useful for the case where the user may want to instantiate
new `Ui` sublayers that also track the scene (by default, sublayers do
*not* apply the same transform as the scene, likely the cause of emilk#5682).
With these changes, the user can have sublayers track the scene like so:

```rust
let scene_layer = ui.layer_id();
let sub_layer = egui::LayerId::new(scene_layer.order, self.id);
ui.ctx().set_sublayer(scene_layer, sub_layer);
let scene_transform = ui.ctx().layer_transform_to_global(scene_layer).unwrap();
ui.ctx().set_transform_layer(sub_layer, scene_transform);
```

### Tested with

- `egui_demo_app` scene example.
- Local `egui_graph` demo example.

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* Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>
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