In the case of paintToCanvas into an accelerated destination canvas, drawing may happen later, so we need to copy the pixel data. ImageBuffer already knows to do this, but this WebGL-specific function does not!
https://bugs.webkit.org/show_bug.cgi?id=77185 was the fix for this for 2D canvas.
Created attachment 185440 [details] patch
http://trac.webkit.org/changeset/141288