canvas: Make 2D context state creation failable and use dom_canvas_backend pref for backend selection (#38310)

Before script just crashed in those cases because IPCSender was dropped,
now we send `None` to tell script about the failure and fail getContext
or registerPainter accordingly.
This PR also unifies `dom_canvas_{backends}_enabled` prefs into
`dom_canvas_backend` which is more flexible in multi-backends scenarios.

Reviewable per commit.

Testing: Added servo specific WPT test.

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/script/dom/offscreencanvas.rs

@@ -135,7 +135,7 @@ impl OffscreenCanvas {
                 _ => None,
             };
         }
-        let context = OffscreenCanvasRenderingContext2D::new(&self.global(), self, can_gc);
+        let context = OffscreenCanvasRenderingContext2D::new(&self.global(), self, can_gc)?;
         *self.context.borrow_mut() = Some(OffscreenRenderingContext::Context2d(Dom::from_ref(
             &*context,
         )));