GTK vs FLTK?

[Juhana Sadeharju <kouhia nic funet fi>]

Hello.
The CinePaint project still wants to move from GTK to FLTK.
Anyone care quickly to compare GTK and FLTK?

FLTK provides an OpenGL overlay, e.g., one can have an image
at background and an OpenGL line/polygon drawing on it.
Is FLTK's overlay the kind of overlay I described?

I have played with GtkGlExt. The overlay support is in the TODO.
Are there ways to circumvent the lack of the overlay support?

Juhana