Auto merge of #20680 - Brody-Eastwood:master, r=emilio

Moved Canvas rendering to a single thread.

<!-- Please describe your changes on the following line: -->

Implements the "Subsequent Steps" part of the [NCSU Canvas rendering project.](https://github.com/servo/servo/wiki/Canvas-rendering-project)

I moved most of the functionality from CanvasPaintThread to CanvasData, so CanvasPaintThread essentially just pulls the info out of the message and calls a method on a particular CanvasData element.

I ran into some awkwardness with the fact that one can only take a single mutable reference from a hashmap, though. DrawImageInOther is not really possible to do with only one reference at a time, so it awkwardly still lives in CanvasPaintThread, basically. I also would've preferred to take the reference at the top as soon as I get the CanvasId, since that looks cleaner than having them all start with "canvas_paint_thread.canvases.get_mut[&canvas_id].unwrap()" but that makes trying to take the second reference for DrawImageInOther fail to compile. I'm definitely open to suggestions on how to make that less gross.

The timed single-canvas drawing improved in performance from around ~2.2ms to around ~1.7ms. Slither.io runs better and doesn't crash, but I'm not having it crash on my copy from before these changes, so I don't know if that's new behavior or not.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13879 and #10381.

<!-- Either: -->
- [X] There are tests for these changes (added in the initial steps)

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20680)
<!-- Reviewable:end -->

components/script/dom/canvasrenderingcontext2d.rs

@@ -371,20 +371,19 @@ impl CanvasRenderingContext2D {
                 None => return Err(Error::InvalidState),
             };
 
-            let (sender, receiver) = ipc::channel(self.global().time_profiler_chan().clone()).unwrap();
-            let msg = CanvasMsg::Canvas2d(Canvas2dMsg::DrawImageInOther(
-                self.ipc_renderer.clone(),
-                self.get_canvas_id(),
-                image_size,
-                dest_rect,
-                source_rect,
-                smoothing_enabled,
-                sender),
-                context.get_canvas_id());
+            let msg = CanvasMsg::Canvas2d(
+                Canvas2dMsg::DrawImageInOther(
+                    self.get_canvas_id(),
+                    image_size,
+                    dest_rect,
+                    source_rect,
+                    smoothing_enabled
+                ),
+                context.get_canvas_id()
+            );
 
             let renderer = context.get_ipc_renderer();
             renderer.send(msg).unwrap();
-            receiver.recv().unwrap();
         };
 
         self.mark_as_dirty();