Allow DataTexture to be used as cube map.

This is an alternative way to accomplish the change proposed in https://github.com/mrdoob/three.js/pull/5380

The way to construct a data texture cube map if this approach is used would be, e.g.:

        var x_neg = new THREE.DataTexture( pixelsSide, 2, 2, THREE.RGBAFormat );
        var x_pos = new THREE.DataTexture( pixelsSide, 2, 2, THREE.RGBAFormat );
        var y_neg = new THREE.DataTexture( pixelsBot, 2, 2, THREE.RGBAFormat );
        var y_pos = new THREE.DataTexture( pixelsTop, 2, 2, THREE.RGBAFormat );
        var z_neg = new THREE.DataTexture( pixelsSide, 2, 2, THREE.RGBAFormat );
        var z_pos = new THREE.DataTexture( pixelsSide, 2, 2, THREE.RGBAFormat );

        var texture = new THREE.Texture(null, new THREE.CubeReflectionMapping(),
                                        THREE.RepeatWrapping, THREE.RepeatWrapping,
                                        THREE.LinearFilter, THREE.LinearFilter,
                                        THREE.RGBAFormat);
        texture.image = [x_pos, x_neg, y_pos, y_neg, z_pos, z_neg];
        texture.needsUpdate = true;