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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