Ensure compatible layout for vertices

Require that these structs have standard layout, which is what we need
to pass them to OpenGL. Inheritence with data members violates the
rules for standard layout so I re-implemented them using templates to
share the code and putting all the data members in each struct.

Change-Id: I19cd0e5518728f2d3e0993b17d15fce7be7e0edb