Some fixes to TexSubImage tests

According to OpenGL ES 2.0 specification it is not possible to use both texture2D and textureCubeMap in one texture unit.

conformance/more/functions/texSubImage2DHTML.html
conformance/more/functions/texSubImage2D.html
conformance/more/functions/copyTexSubImage2D.html

should all be using gl.activeTexture appropriate to switch between using the texture bound to GL_TEXTURE_2D, and the texture bound to GL_TEXTURE_CUBE_MAP.

And a nit: conformance/more/textures/texture-size.html passes fine, but green and blue are mixed up...