Fix some warnings from Clang trunk in an overly aggressive manner

The main reason I added the dummy parameter is that otherwise it can't infer the return type.  AFAICT, this is what necessitated the ugly `template<class T> T& GetUidData()` that actually just returned one type.  I really don't like that, so I went with the least ugly obvious alternative.

I suppose I could change it to two methods, one for pixel and one for vertex...