Fully support <input type=color> (#36992)

This change adds a shadow-tree widget for `<input type=color>` elements.
It also involves some changes to the way layout interacts with the DOM,
because currently all `input` and `textarea` elements are rendered as
plain text and their descendants are ignored. This obviously doesn't
work for `<input type={color, date, range, etc}>`.


![image](https://github.com/user-attachments/assets/4f16c3b0-1f79-4095-b19d-1153f5853dd5)

<details><summary>HTML used for the screenshot above</summary>

```html
<input type=color>
```

</details>



Testing: I doubt that this affects WPT tests, because the appearance and
behaviour of the widget is almost entirely unspecified.

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

ports/servoshell/desktop/dialog.rs

@@ -12,9 +12,9 @@ use log::warn;
 use servo::ipc_channel::ipc::IpcSender;
 use servo::servo_geometry::DeviceIndependentPixel;
 use servo::{
-    AlertResponse, AuthenticationRequest, ConfirmResponse, FilterPattern, PermissionRequest,
-    PromptResponse, SelectElement, SelectElementOption, SelectElementOptionOrOptgroup,
-    SimpleDialog,
+    AlertResponse, AuthenticationRequest, ColorPicker, ConfirmResponse, FilterPattern,
+    PermissionRequest, PromptResponse, RgbColor, SelectElement, SelectElementOption,
+    SelectElementOptionOrOptgroup, SimpleDialog,
 };
 
 pub enum Dialog {
@@ -43,6 +43,11 @@ pub enum Dialog {
         maybe_prompt: Option<SelectElement>,
         toolbar_offset: Length<f32, DeviceIndependentPixel>,
     },
+    ColorPicker {
+        current_color: egui::Color32,
+        maybe_prompt: Option<ColorPicker>,
+        toolbar_offset: Length<f32, DeviceIndependentPixel>,
+    },
 }
 
 impl Dialog {
@@ -119,6 +124,22 @@ impl Dialog {
         }
     }
 
+    pub fn new_color_picker_dialog(
+        prompt: ColorPicker,
+        toolbar_offset: Length<f32, DeviceIndependentPixel>,
+    ) -> Self {
+        let current_color = egui::Color32::from_rgb(
+            prompt.current_color().red,
+            prompt.current_color().green,
+            prompt.current_color().blue,
+        );
+        Dialog::ColorPicker {
+            current_color,
+            maybe_prompt: Some(prompt),
+            toolbar_offset,
+        }
+    }
+
     pub fn update(&mut self, ctx: &egui::Context) -> bool {
         match self {
             Dialog::File {
@@ -485,6 +506,50 @@ impl Dialog {
                     maybe_prompt.take().unwrap().submit();
                 }
 
+                is_open
+            },
+            Dialog::ColorPicker {
+                current_color,
+                maybe_prompt,
+                toolbar_offset,
+            } => {
+                let Some(prompt) = maybe_prompt else {
+                    // Prompt was dismissed, so the dialog should be closed too.
+                    return false;
+                };
+                let mut is_open = true;
+
+                let mut position = prompt.position();
+                position.min.y += toolbar_offset.0 as i32;
+                position.max.y += toolbar_offset.0 as i32;
+                let area = egui::Area::new(egui::Id::new("select-window"))
+                    .fixed_pos(egui::pos2(position.min.x as f32, position.max.y as f32));
+
+                let modal = Modal::new("select_element_picker".into()).area(area);
+                modal.show(ctx, |ui| {
+                    egui::widgets::color_picker::color_picker_color32(
+                        ui,
+                        current_color,
+                        egui::widgets::color_picker::Alpha::Opaque,
+                    );
+
+                    ui.add_space(10.);
+
+                    if ui.button("Dismiss").clicked() {
+                        is_open = false;
+                        prompt.select(None);
+                    }
+                    if ui.button("Select").clicked() {
+                        is_open = false;
+                        let selected_color = RgbColor {
+                            red: current_color.r(),
+                            green: current_color.g(),
+                            blue: current_color.b(),
+                        };
+                        prompt.select(Some(selected_color));
+                    }
+                });
+
                 is_open
             },
         }