Auto merge of #6183 - ecoal95:webglcontextattributes, r=nox

r? @jdm

I couldn't add the `getContextAttributes` method since `CodegenRust`
doesn't know how to return a dictionary value, I'll take a look at it ASAP.

I think the helper functions can return directly the renderer, since they're used just for that, but I wanted to hear your opinions about this.

By the way I'm interested in adding more serious tests for WebGL, and I think the [khronos conformance suit](https://github.com/KhronosGroup/WebGL/tree/master/conformance-suites/1.0.3) should be the best option.

Should I try to integrate it in wpt, or making a `tests/webgl` directory (or similar) inside the servo tree? (Maybe this question should be for @Ms2ger)

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components/script/dom/canvasrenderingcontext2d.rs

@@ -214,7 +214,11 @@ impl CanvasRenderingContext2D {
         let msg = if self.canvas.root().r() == canvas {
             CanvasMsg::Canvas2d(Canvas2dMsg::DrawImageSelf(image_size, dest_rect, source_rect, smoothing_enabled))
         } else { // Source and target canvases are different
-            let context = canvas.get_2d_context().root();
+            let context = match canvas.get_or_init_2d_context() {
+                Some(context) => context.root(),
+                None => return Err(InvalidState),
+            };
+
             let renderer = context.r().get_renderer();
             let (sender, receiver) = channel::<Vec<u8>>();
             // Reads pixels from source image