Add Init() method to Renderer

- moved the GL scissor disable to Init() because
  until setVideoMode() is called there may not be
  a valid GL context. This can happen, for example,
  in OS X if your PolyCocoaView isn't initialized
  from a nib and instead from code. Calling GL
  without a valid context can crash.
- Replaced calls to initOSSpecific() with calls to
  Init().