Inject DearImGUI into an existing VTK application.

A very closely related project is phcerdan's https://github.com/phcerdan/vtkImGuiAdapter. The two are slightly different in that:

1. With vtkDearImGUIInjector,

   a. you do not have to take care of ImGUI contexts yourself.

   b. no need to write an event loop.

2. With vtkImGUIAdapter,

   a. you are responsible for ensuring the sdl2 window is setup and the ImGUI context is initialized.

   b. you must write an event loop in your application, ensure events are processed and ensure NewFrame/EndFrame are called.