1. Fixed TimerId type. vtkRenderWindowInteractor defines the TimerId
type as int. vtkContextInteractorStyle now uses the same type.

2. Fixed timer creation. OnSceneModifed() was repeatedly creating new
timers even when previous one hadn't timed out yet. Fixed that. Also,
RenderNow() ensures that old timers are destroyed.

3. RenderNow() now uses Interactor->Render() rather than
Interactor->GetRenderWindow()->Render(). vtkRenderWindowInteractor has
API to control how and if an interaction should trigger a render on the
window. Calling RenderWindow->Render() bypasses all that logic resulting
is mismatch between how interactions work in Render views and Chart views.
This fixes that issue.

Change-Id: I58798615070d97ba9a00ccc21919a5d518bdac22