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.
Let's please continue to use the 1x2 variant for the TEXTURE_2D tests, and only use the new 2x2 variant for the cube map tests.