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 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.