vtkCocoaRenderWindow: check if Initialized before calling Initialize()
Since MR !9845 (merged), we've started running into OpenGL issues in a ParaView-based application when running on Mac. Some images wouldn't render and glBindFramebuffer was returning invalid operation. The FBO in question appeared to be valid (and in fact a prior call to glBindFramebuffer on that fbo was successful). I finally found out that vtkCocoaRenderWindow::Start had been called again which calls vtkCocoaRenderWindow::Initialize. Previously, vtkCocoaRenderWindow::Initialize was called by vtkOpenGLRenderWindow which only calls Initialize if it hasn't been already, so this MR adds the same check and this fixes the issues for us.