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.

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

components/shared/canvas/lib.rs

@@ -18,7 +18,7 @@ pub mod webgl;
 
 pub enum ConstellationCanvasMsg {
     Create {
-        sender: Sender<(CanvasId, ImageKey)>,
+        sender: Sender<Option<(CanvasId, ImageKey)>>,
         size: Size2D<u64>,
     },
     Exit(Sender<()>),