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.
Yes, that sounds good. This is more reason to split up the main loop into two separate cases, one which tests the 2D texture case and one which tests the cube map case.