Install libgtkglext1-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libgtkglext1-doc
Description:
OpenGL Extension to GTK+ (documentation)
GtkGLExt provides the GDK objects to support OpenGL rendering in GTK+, and GtkWidget API add-ons to make GTK+ widgets OpenGL-capable. . This package contains documentation for libgtkglext.
Homepage: http://projects.gnome.org/gtkglext/
Version: 1.2.0-8
Section: universe/doc