canvas: Add OffscreenCanvas 'transferToImageBitmap' method (#37880)

Follow the HTML speficication and add missing 'transferToImageBitmap'
method to OffscreenCanvas interface.

https://html.spec.whatwg.org/multipage/#dom-offscreencanvas-transfertoimagebitmap

Testing: Improvements in the following tests
- html/canvas/offscreen/compositing/2d.composite.grid*
- html/canvas/offscreen/fill-and-stroke-styles/2d.gradient*
- html/canvas/offscreen/manual/the-offscreen-canvas/offscreencanvas*
- html/canvas/offscreen/reset/2d.reset*
- html/canvas/offscreen/text/2d.text*

Fixes (partially): #34111

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>

components/script/canvas_context.rs

@@ -39,6 +39,11 @@ pub(crate) trait CanvasContext {
 
     fn resize(&self);
 
+    // Resets the backing bitmap (to transparent or opaque black) without the
+    // context state reset.
+    // Used by OffscreenCanvas.transferToImageBitmap.
+    fn reset_bitmap(&self);
+
     /// Returns none if area of canvas is zero.
     ///
     /// In case of other errors it returns cleared snapshot
@@ -145,6 +150,21 @@ impl CanvasContext for RenderingContext {
         }
     }
 
+    fn reset_bitmap(&self) {
+        match self {
+            RenderingContext::Placeholder(offscreen_canvas) => {
+                if let Some(context) = offscreen_canvas.context() {
+                    context.reset_bitmap()
+                }
+            },
+            RenderingContext::Context2d(context) => context.reset_bitmap(),
+            RenderingContext::WebGL(context) => context.reset_bitmap(),
+            RenderingContext::WebGL2(context) => context.reset_bitmap(),
+            #[cfg(feature = "webgpu")]
+            RenderingContext::WebGPU(context) => context.reset_bitmap(),
+        }
+    }
+
     fn get_image_data(&self) -> Option<Snapshot> {
         match self {
             RenderingContext::Placeholder(offscreen_canvas) => {
@@ -257,6 +277,12 @@ impl CanvasContext for OffscreenRenderingContext {
         }
     }
 
+    fn reset_bitmap(&self) {
+        match self {
+            OffscreenRenderingContext::Context2d(context) => context.reset_bitmap(),
+        }
+    }
+
     fn get_image_data(&self) -> Option<Snapshot> {
         match self {
             OffscreenRenderingContext::Context2d(context) => context.get_image_data(),