- init 'GL' handle of rendering classes in MapRenderer.onSurfaceCreated, at this point handle is safe to use, i.e. GL context is available - all LayerRenderer share one static 'GL' field now
- init 'GL' handle of rendering classes in MapRenderer.onSurfaceCreated, at this point handle is safe to use, i.e. GL context is available - all LayerRenderer share one static 'GL' field now