Make all lines and rectangles crisp (#5518)

* Merge this first: https://github.com/emilk/egui/pull/5517

This aligns all rectangles and (horizontal or vertical) line segments to
the physical pixel grid in the `epaint::Tessellator`, making these
shapes appear crisp everywhere.

* Closes https://github.com/emilk/egui/issues/5164
* Closes https://github.com/emilk/egui/issues/3667

This undoes a lot of the explicit, egui-side aligning added in:
* https://github.com/emilk/egui/pull/4943

The new approach has several benefits over the old one:

* It is done automatically by epaint, so it is applied to everything (no
longer opt-in)
* It is applied after any layer transforms (so it always works)
* It makes line segments crisper on high-DPI screens
* All filled rectangles now has sides that end on pixel boundaries

crates/epaint/src/shape_transform.rs

@@ -21,7 +21,7 @@ pub fn adjust_colors(
         }
 
         Shape::LineSegment { stroke, points: _ } => {
-            adjust_color_mode(&mut stroke.color, adjust_color);
+            adjust_color(&mut stroke.color);
         }
 
         Shape::Path(PathShape {