Added render methods to interface and fixed renderAllExcept for WavefrontObjects

Fixed two little bugs in the WavefrontObject render files.
Firstly I fixed the renderAllExcept method so that it will not render the GroupObjects with given GroupObject names but render all other GroupObjects, like you would expect it to do.
Secondly I added the renderAllExcept method and the renderOnly method to the IModelCustom interface, so they can be used by Objects using that interface.