New triangle collapse arrow for large collapsible header (#2920)

### What

This PR implements the proper Figma-based collapsing arrow (with
"linearised" rounded corners) and uses it for the large collapsible
headers in the selection panel. This PR also fixes the vertical optical
alignment of the text within these headers.

Partially addresses #2738 

Zoom 150%:
<img width="438" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/b51a41a3-7f77-4e56-8639-c3428a124829">


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2920) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2920)
- [Docs
preview](https://rerun.io/preview/pr%3Aantoine%2Fcollapsing-triangle/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Aantoine%2Fcollapsing-triangle/examples)

crates/re_ui/src/lib.rs

@@ -52,6 +52,7 @@ use std::{ops::RangeInclusive, sync::Arc};
 use parking_lot::Mutex;
 
 use crate::list_item::ListItem;
+use egui::emath::Rot2;
 use egui::{pos2, Align2, Color32, Mesh, NumExt, Rect, Shape, Vec2};
 
 #[derive(Clone)]
@@ -576,12 +577,16 @@ impl ReUi {
                     egui::Vec2::splat(icon_width),
                 );
                 let icon_response = header_response.clone().with_new_rect(icon_rect);
-                egui::collapsing_header::paint_default_icon(ui, openness, &icon_response);
+                Self::paint_collapsing_triangle(ui, openness, &icon_response);
 
                 let visuals = ui.style().interact(&header_response);
 
+                let optical_vertical_alignment = 0.5; // improves perceived vertical alignment
                 let text_pos = icon_response.rect.right_center()
-                    + egui::vec2(space_after_icon, -0.5 * galley.size().y);
+                    + egui::vec2(
+                        space_after_icon,
+                        -0.5 * galley.size().y + optical_vertical_alignment,
+                    );
                 ui.painter()
                     .galley_with_color(text_pos, galley, visuals.text_color());
 
@@ -607,6 +612,46 @@ impl ReUi {
         });
     }
 
+    /// Paint a collapsing triangle with rounded corners.
+    ///
+    /// Alternative to [`egui::collapsing_header::paint_default_icon`].
+    pub fn paint_collapsing_triangle(ui: &mut egui::Ui, openness: f32, response: &egui::Response) {
+        let visuals = ui.style().interact(response);
+
+        let rect = response.rect;
+        let extent = response.rect.width().min(response.rect.height());
+
+        // Normalised in [0, 1]^2 space.
+        // Note on how these coords have been computed: https://github.com/rerun-io/rerun/pull/2920
+        // Discussion on the future of icons:  https://github.com/rerun-io/rerun/issues/2960
+        let mut points = vec![
+            pos2(0.80387, 0.470537),
+            pos2(0.816074, 0.5),
+            pos2(0.80387, 0.529463),
+            pos2(0.316248, 1.017085),
+            pos2(0.286141, 1.029362),
+            pos2(0.257726, 1.017592),
+            pos2(0.245118, 0.987622),
+            pos2(0.245118, 0.012378),
+            pos2(0.257726, -0.017592),
+            pos2(0.286141, -0.029362),
+            pos2(0.316248, -0.017085),
+            pos2(0.80387, 0.470537),
+        ];
+
+        use std::f32::consts::TAU;
+        let rotation = Rot2::from_angle(egui::remap(openness, 0.0..=1.0, 0.0..=TAU / 4.0));
+        for p in &mut points {
+            *p = rect.center() + rotation * (*p - pos2(0.5, 0.5)) * extent;
+        }
+
+        ui.painter().add(Shape::convex_polygon(
+            points,
+            visuals.fg_stroke.color,
+            egui::Stroke::NONE,
+        ));
+    }
+
     /// Workaround for putting a label into a grid at the top left of its row.
     #[allow(clippy::unused_self)]
     pub fn grid_left_hand_label(&self, ui: &mut egui::Ui, label: &str) -> egui::Response {