Auto merge of #6083 - ecoal95:layerize-canvas, r=pcwalton

I've done a bit of job to get this done. Right now readback is still used, but we have a `LayerId` -> `CanvasRenderer` map on the paint task, that we can use to get rid of that.

I'd want review, to see if this is a good approach (I know it's not the initial `CanvasId` -> renderer approach, but it's pretty similar, since a canvas involves a `PaintLayer`).

I had to do a bit of refactoring to avoid cyclic dependencies between canvas and gfx. I'd want you to review them too.

It's mergeable and doesn't break any tests :P

Some of my main concerns:
* Does the canvas render really need to be behind an `Arc<Mutex<T>>`?
* I can't clone a `NativeSurface` right now (that's why the `SendNativeSurface()` msg is unimplemented in the WebGL task). It should be easy to add that to rust-layers, supposing the caller is responsible to mark it as non-leaking, any reason to not do it?

cc @jdm @pcwalton

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components/script/dom/bindings/trace.rs

@@ -34,8 +34,8 @@ use dom::bindings::refcounted::Trusted;
 use dom::bindings::utils::{Reflectable, Reflector, WindowProxyHandler};
 use script_task::ScriptChan;
 
-use canvas::canvas_paint_task::{CanvasGradientStop, LinearGradientStyle, RadialGradientStyle};
-use canvas::canvas_paint_task::{LineCapStyle, LineJoinStyle, CompositionOrBlending};
+use canvas_traits::{CanvasGradientStop, LinearGradientStyle, RadialGradientStyle};
+use canvas_traits::{LineCapStyle, LineJoinStyle, CompositionOrBlending};
 use cssparser::RGBA;
 use encoding::types::EncodingRef;
 use geom::matrix2d::Matrix2D;

components/script/dom/canvasgradient.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use cssparser::RGBA;
-use canvas::canvas_paint_task::{CanvasGradientStop, FillOrStrokeStyle, LinearGradientStyle, RadialGradientStyle};
+use canvas_traits::{CanvasGradientStop, FillOrStrokeStyle, LinearGradientStyle, RadialGradientStyle};
 use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::CanvasGradientBinding;
 use dom::bindings::codegen::Bindings::CanvasGradientBinding::CanvasGradientMethods;

components/script/dom/canvasrenderingcontext2d.rs

@@ -28,10 +28,10 @@ use geom::point::Point2D;
 use geom::rect::Rect;
 use geom::size::Size2D;
 
-use canvas::canvas_msg::{CanvasMsg, Canvas2dMsg, CanvasCommonMsg};
-use canvas::canvas_paint_task::{CanvasPaintTask, FillOrStrokeStyle};
-use canvas::canvas_paint_task::{LinearGradientStyle, RadialGradientStyle};
-use canvas::canvas_paint_task::{LineCapStyle, LineJoinStyle, CompositionOrBlending};
+use canvas_traits::{CanvasMsg, Canvas2dMsg, CanvasCommonMsg};
+use canvas_traits::{FillOrStrokeStyle, LinearGradientStyle, RadialGradientStyle};
+use canvas_traits::{LineCapStyle, LineJoinStyle, CompositionOrBlending};
+use canvas::canvas_paint_task::CanvasPaintTask;
 
 use net_traits::image::base::Image;
 use net_traits::image_cache_task::ImageCacheChan;

components/script/dom/htmlcanvaselement.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use canvas::canvas_msg::CanvasMsg;
+use canvas_traits::CanvasMsg;
 use dom::attr::Attr;
 use dom::attr::AttrHelpers;
 use dom::bindings::codegen::Bindings::HTMLCanvasElementBinding;

components/script/dom/webglrenderingcontext.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use canvas::webgl_paint_task::WebGLPaintTask;
-use canvas::canvas_msg::{CanvasMsg, CanvasWebGLMsg, CanvasCommonMsg};
+use canvas_traits::{CanvasMsg, CanvasWebGLMsg, CanvasCommonMsg};
 use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding;
 use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::{ WebGLRenderingContextMethods, WebGLRenderingContextConstants};
 use dom::bindings::global::{GlobalRef, GlobalField};