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_lib/tests/snapshots/demos/Font Book.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:157424625dbb855e116a29a008172f29697a37b03512ef06a2e92e502d5e4128
-size 101816
+oid sha256:73429ec71d1ae05518352f1a99304f55647dac362092c9cd5618f709b9ba0d11
+size 99950

crates/egui_demo_lib/tests/snapshots/demos/ID Test.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:2027f21fdb132542e01fc592d3150aa876cf2ebb4a4268b8c170ff4f523657f0
-size 114074
+oid sha256:d14571fdd7602ea4924d04725008433e1bb53596d8a279f310ddc6c3231b6d32
+size 114106

crates/egui_demo_lib/tests/snapshots/demos/Misc Demos.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:6e00e1dd95278a003c383f5a3f16d25ee4943073171d862db50cb17e90e421bc
-size 60272
+oid sha256:2909f098b5edacefef1b5c4d81982b0c84ebd27f896934f0bef92ceb283bbe79
+size 60288

crates/egui_demo_lib/tests/snapshots/demos/Panels.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:fc1d270e0171cc055fe99579d3f7ab52c63783c793c59aaf8c82027bf1dd3ab8
-size 216484
+oid sha256:1cd58e9912d1db051c93a9b2ea340f3f6dfe64d38ed54aa1b8292bc6f1bf970c
+size 215502

crates/egui_demo_lib/tests/snapshots/demos/Scrolling.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:10e4658168d6a93779463ebb81520f821cbf6eac59391c60d30f5d40ebdf910d
-size 152623
+oid sha256:fc28830c402d3aa861a220cfc600c3ce2d7b8ca77307691b3f844a34f27afd1f
+size 151376

crates/egui_demo_lib/tests/snapshots/demos/Table.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:66f35217b0becce12e251c4e7063c31ac92e080340a0318891771fdff54593db
-size 67388
+oid sha256:ca08bb7cd0be0b5a9d10833e5c9b65e889f78a5a8056df69d6b0006c3a228227
+size 66050

crates/egui_demo_lib/tests/snapshots/demos/Text Layout.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:29e23e0e45a6e557e569358c89a34dc768ff0b33b47112f864ae8ed119ac6e6e
-size 81119
+oid sha256:121c6d5ba129439d2d747da8e505cfff04a023c28d9a0e3aca324d77de1ef418
+size 79186

crates/egui_demo_lib/tests/snapshots/demos/Tooltips.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:dde445e82732c45f84acbfa83761348b9b89dd2a4fec500ba5124b0c1e58dd29
-size 63060
+oid sha256:3e44cef45d27ddd64b0d2f6de0b7005846147bce54a9b8b1223ba9a0a02c2416
+size 62830

crates/egui_demo_lib/tests/snapshots/demos/Window Resize Test.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:f9c8eecaedce2bbcbdc4b7975a349b9f986463f189041c7ad6bed64809f18bcd
-size 445367
+oid sha256:90c4ea93f234aded298ddeeb98e2d6a0566f07efcdaf5c990e8a47a6533db516
+size 446483