Separate class for rendering statistics

[liaxim]

Per @NolaDonato: I suggest that we make a separate class for rendering statistics instead of adding these functions to the renderer. In the future we will probably want to keep track of more things like the number of state changes, the number of shader changes, average batch size, etc. Since we are refactoring it would be more open-ended if we collected statistics and processed them in a separate class.

Maybe this class can build on top of the existing (generic) GVRStatsLine.