Make tex-image-and-sub-image-2d-with-webgl-canvas-* cover TEXTURE_CUBE_MAP.

Make tex-image-and-sub-image-2d-with-webgl-canvas-* cover TEXTURE_CUBE_MAP.

Improve tex-image-and-sub-image-2d-with-webgl-canvas-* to check http://crrev.com/735623003/
Change from setCanvasTo1x2 to setCanvasTo2x2 because cube map allows only square source.
While I appreciate the fact that you're trying to share as much code as possible, modifying this internal state in this way makes it really difficult to understand the control flow. Could you please consider just copying the function and having two parts to the test, the first one testing TEXTURE_2D and the second testing TEXTURE_CUBE_MAP targets?