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.
To achieve it, I changes this tests a bit more.
I need to change fshader to draw 6 faces of cube map, and draw 6 times to canvas and check 6 times. Additional uniform will control which face is drawn.
Does it sound good?