[vtkusers] toolkit implementation

[Doug Henry]

I'm wondering if there are resources (documentation) available that
dicuss the process of implementing a renderer for an unsupported
toolkit?  I found a vtk package for FLTK which looks like I would
expect, but I'm unsure of some things that are being done.  For
example, vtkOpenGLRenderWindow is subclassed to create
vtkFLTKOpenGLRenderWindow, but there are some mystery files created
through the use of VTK_MAKE_INSTANTIATOR cmake macro.  Anything that
helps to explain the minimal set of things that need implemented would
be extremely helpful.

Thanks,

Doug