Add RectShape::blur_width to implement shadows (#4267)

This is mostly a refactor, but has some performance benefits:
* We (re)use the same tessellator as for everything else, leading to
less allocations
* We cull shapes before rendering them

Adding `RectShape::blur_width` means it can also be used for other
effects, such as glow.

crates/egui/src/containers/frame.rs

@@ -291,9 +291,8 @@ impl Frame {
         if shadow == Default::default() {
             frame_shape
         } else {
-            let shadow = shadow.tessellate(outer_rect, rounding);
-            let shadow = Shape::Mesh(shadow);
-            Shape::Vec(vec![shadow, frame_shape])
+            let shadow = shadow.as_shape(outer_rect, rounding);
+            Shape::Vec(vec![Shape::from(shadow), frame_shape])
         }
     }
 }

crates/egui/src/widgets/image.rs

@@ -762,6 +762,7 @@ pub fn paint_texture_at(
                 rounding: options.rounding,
                 fill: options.tint,
                 stroke: Stroke::NONE,
+                blur_width: 0.0,
                 fill_texture_id: texture.id,
                 uv: options.uv,
             });

crates/egui/src/widgets/slider.rs

@@ -739,14 +739,8 @@ impl<'a> Slider<'a> {
                     };
                     let v = v + Vec2::splat(visuals.expansion);
                     let rect = Rect::from_center_size(center, 2.0 * v);
-                    ui.painter().add(epaint::RectShape {
-                        fill: visuals.bg_fill,
-                        stroke: visuals.fg_stroke,
-                        rect,
-                        rounding: visuals.rounding,
-                        fill_texture_id: Default::default(),
-                        uv: Rect::ZERO,
-                    });
+                    ui.painter()
+                        .rect(rect, visuals.rounding, visuals.bg_fill, visuals.fg_stroke);
                 }
             }
         }