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.
It looks to me like this only samples one of the cube map's faces, where it should be parameterized to sample the face that was most recently uploaded. Is that correct? If so, please generalize the test so that it verifies all uploaded faces.