Added FPS counter to the GameState.

Game now calls GameState::DoRender. The DoRender calls Render and then
draws fps counter to the top right of the screen.
TODO: DoRender falsely assumes that the viewport and projection are good for
text drawing. This needs to be fixed at some point.

Added:
Renderer::GetTextWidth
Renderer::GetTextHeight

Added constants Color::Balck and Color::White.