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>

components/shared/embedder/lib.rs

@@ -364,16 +364,8 @@ pub enum EmbedderMsg {
     ShutdownComplete,
     /// Request to display a notification.
     ShowNotification(Option<WebViewId>, Notification),
-    /// Indicates that the user has activated a `<select>` element.
-    ///
-    /// The embedder should respond with the new state of the `<select>` element.
-    ShowSelectElementMenu(
-        WebViewId,
-        Vec<SelectElementOptionOrOptgroup>,
-        Option<usize>,
-        DeviceIntRect,
-        IpcSender<Option<usize>>,
-    ),
+    /// Request to display a form control to the embedder.
+    ShowFormControl(WebViewId, DeviceIntRect, FormControl),
     /// Inform the embedding layer that a JavaScript evaluation has
     /// finished with the given result.
     FinishJavaScriptEvaluation(
@@ -389,6 +381,18 @@ impl Debug for EmbedderMsg {
     }
 }
 
+#[derive(Deserialize, Serialize)]
+pub enum FormControl {
+    /// Indicates that the user has activated a `<select>` element.
+    SelectElement(
+        Vec<SelectElementOptionOrOptgroup>,
+        Option<usize>,
+        IpcSender<Option<usize>>,
+    ),
+    /// Indicates that the user has activated a `<input type=color>` element.
+    ColorPicker(RgbColor, IpcSender<Option<RgbColor>>),
+}
+
 /// Filter for file selection;
 /// the `String` content is expected to be extension (e.g, "doc", without the prefixing ".")
 #[derive(Clone, Debug, Deserialize, Serialize)]
@@ -921,3 +925,10 @@ pub enum JavaScriptEvaluationError {
     /// value into a [`JSValue`].
     SerializationError,
 }
+
+#[derive(Clone, Copy, Debug, Deserialize, Serialize)]
+pub struct RgbColor {
+    pub red: u8,
+    pub green: u8,
+    pub blue: u8,
+}