Add unbindImageTexture

The parameters `level, layered, layer, access & format` seem to be irrelevant for unbinding, according to the spec, but must be from valid ranges.

Also, remove unnecessary call to bind in bindImageTexture as the texture to bind is identified via its ID.
Nice overspecification from the OpenGL API, but I think this should be clearly and easily checkable.
Do you have an URL that states this behavior?