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GwyGradientEditor is the application OpenGL material editor.  The interface
is currently extremely simple: gwy_app_gl_material_editor() invokes
the editor (or brings it forward) and then it's user-controlled.
void
gwy_app_gl_material_editor (void);
Creates or presents OpenGL material editor.
Material editor is singleton, therefore if it doesn't exist, this function
creates and displays it.  If it already exists, it simply calls
gtk_window_present() on the existing instance.  It exists until it's closed
by user.