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

Moved Canvas rendering to a single thread.

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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.

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- [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.

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- [X] There are tests for these changes (added in the initial steps)

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components/constellation/constellation.rs

@@ -330,8 +330,8 @@ pub struct Constellation<Message, LTF, STF> {
     /// A channel through which messages can be sent to the webvr thread.
     webvr_chan: Option<IpcSender<WebVRMsg>>,
 
-    /// An Id for the next canvas to use.
-    canvas_id: CanvasId,
+    /// A channel through which messages can be sent to the canvas paint thread.
+    canvas_chan: IpcSender<CanvasMsg>,
 }
 
 /// State needed to construct a constellation.
@@ -630,7 +630,7 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 }),
                 webgl_threads: state.webgl_threads,
                 webvr_chan: state.webvr_chan,
-                canvas_id: CanvasId(0),
+                canvas_chan: CanvasPaintThread::start(),
             };
 
             constellation.run();
@@ -2294,11 +2294,25 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
             &mut self,
             size: &Size2D<i32>,
             response_sender: IpcSender<(IpcSender<CanvasMsg>, CanvasId)>) {
-        self.canvas_id.0 += 1;
         let webrender_api = self.webrender_api_sender.clone();
-        let sender = CanvasPaintThread::start(*size, webrender_api,
-                                              opts::get().enable_canvas_antialiasing, self.canvas_id.clone());
-        if let Err(e) = response_sender.send((sender, self.canvas_id.clone())) {
+        let sender = self.canvas_chan.clone();
+        let (canvas_id_sender, canvas_id_receiver) = ipc::channel::<CanvasId>().expect("ipc channel failure");
+
+        if let Err(e) = sender.send(
+            CanvasMsg::Create(
+                canvas_id_sender,
+                *size,
+                webrender_api,
+                opts::get().enable_canvas_antialiasing
+            )
+        ) {
+            return warn!("Create canvas paint thread failed ({})", e);
+        }
+        let canvas_id = match canvas_id_receiver.recv() {
+            Ok(canvas_id) => canvas_id,
+            Err(e) => return warn!("Create canvas paint thread id response failed ({})", e),
+        };
+        if let Err(e) = response_sender.send((sender, canvas_id.clone())) {
             warn!("Create canvas paint thread response failed ({})", e);
         }
     }