WebGLRenderingContextBase::markContextChanged and WebGLRenderingContextBase::markContextChangedAndNotifyCanvasObserver dispatch too many notifications:

- markContextChangedAndNotifyCanvasObserver doesn't take into account whether a user-level framebuffer is bound. If one is, then it shouldn't notify.
- markContextChanged calls "htmlCanvas()->clearCopiedImage()" and "renderBox->contentChanged(CanvasPixelsChanged)" for every draw call - this seems expensive.

dino@ indicated in discussion on Slack that with the addition of WebGLRenderingContextBase::prepareForDisplay in Bug 212461, at least some of these notifications can be done once per frame, at the end of the frame, rather than on each operation that might touch the WebGL back buffer.