Rework Window margins and set clip_rect_margin to zero (#7725)

* Follows https://github.com/emilk/egui/pull/7722
* Part of https://github.com/emilk/egui/issues/5605
* Closes https://github.com/emilk/egui/issues/3385

## What
Sets `clip_rect_margin` to zero, and moves the margin of `Window`s with
`ScrollAreas`, so that the scroll bars are now on the very edge of the
windows they are in.

Windows with a bulit-in scroll area now lets the content go all the way
to the edges (left image).
However, if you just manually add a `ScrollArea` to a `Window`, you
won't get this effect (right image).
<img width="763" height="345" alt="Screenshot 2026-05-18 at 22 04 01"
src="https://github.com/user-attachments/assets/e41cdfcb-b0a6-4e5e-9691-d132a602d6a7"
/>

## Required
* #7803
* #7804
* #7805
* #7806
* https://github.com/emilk/egui/pull/7807
* https://github.com/emilk/egui/pull/7808

crates/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -102,7 +102,7 @@ impl Custom3d {
 
 impl crate::DemoApp for Custom3d {
     fn demo_ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
-        // TODO(emilk): Use `ScrollArea::inner_margin`
+        // TODO(emilk): Use `ScrollArea::content_margin`
         egui::CentralPanel::default().show_inside(ui, |ui| {
             egui::ScrollArea::both().auto_shrink(false).show(ui, |ui| {
                 ui.horizontal(|ui| {