Drawing a 2D canvas to itself with Accelerated2dCanvasEnabled causes rendering artifacts

Attaching an example to demonstrate the issue. The bug seems to be caused by undefined behavior in OpenGL when using the same texture as a source and a destination. I'm working on a fix.

The use case should be covered by these W3C tests too, although they passed when I tried them:

http://w3c-test.org/submissions/539/2dcontext/drawing-images-to-the-canvas/2d.drawImage.self.1.html
http://w3c-test.org/submissions/539/2dcontext/drawing-images-to-the-canvas/2d.drawImage.self.2.html