[debug_layers] Validation, LineSoup visualization, and UI toggles

- Introduce the VALIDATION layer which runs a watertightness test on
  LineSoup buffer contents.
- Add debug visualization layers for LineSoup endpoints and validation
  test errors.
- Add DebugLayers options to toggle individual layers.
- Add with_winit key bindings to toggle individual layers.

examples/with_winit/src/lib.rs

@@ -196,6 +196,10 @@ fn run(
     }
     let mut prev_scene_ix = scene_ix - 1;
     let mut modifiers = ModifiersState::default();
+
+    #[cfg(feature = "debug_layers")]
+    let mut debug = vello::DebugLayers::none();
+
     #[allow(deprecated)]
     event_loop
         .run(move |event, event_loop| match event {
@@ -301,6 +305,22 @@ fn run(
                                                 },
                                             );
                                         }
+                                        #[cfg(feature = "debug_layers")]
+                                        "1" => {
+                                            debug.toggle(vello::DebugLayers::BOUNDING_BOXES);
+                                        }
+                                        #[cfg(feature = "debug_layers")]
+                                        "2" => {
+                                            debug.toggle(vello::DebugLayers::LINESOUP_SEGMENTS);
+                                        }
+                                        #[cfg(feature = "debug_layers")]
+                                        "3" => {
+                                            debug.toggle(vello::DebugLayers::LINESOUP_POINTS);
+                                        }
+                                        #[cfg(feature = "debug_layers")]
+                                        "4" => {
+                                            debug.toggle(vello::DebugLayers::VALIDATION);
+                                        }
                                         _ => {}
                                     }
                                 }
@@ -444,6 +464,8 @@ fn run(
                             width,
                             height,
                             antialiasing_method,
+                            #[cfg(feature = "debug_layers")]
+                            debug,
                         };
                         scene.reset();
                         let mut transform = transform;