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/emath/src/gui_rounding.rs

@@ -116,6 +116,7 @@ impl GuiRounding for crate::Vec2 {
         )
     }
 
+    // This doesn't really make sense for a Vec2, but 🤷‍♂️
     #[inline]
     fn round_to_pixel_center(self, pixels_per_point: f32) -> Self {
         Self::new(