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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Remove insaccessible history states
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Update to smallbitvec 2.1
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Implement gl.getParameter(gl.ALIASED_LINE_WIDTH_RANGE)
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Implement gl.getParameter(gl.ALIASED_LINE_WIDTH_RANGE)
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It doesn't actually matter because this is only called from
WebGLRenderingContext::BindTexture, which already checks the target,
but better be safe than sorry.
NCSU Canvas Rendering Project Initial Steps
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Implements the initial steps from:
https://github.com/servo/servo/wiki/Canvas-rendering-project
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Use higher level handles from JS bindings
…reak servo
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