Refactor drawing small circles: add fl_draw_circle()

This method can be used to draw small circles as part of the GUI.
It is independent of the current scheme.

Very small circles are approximated by drawing several rectangles.

FL/fl_draw.H

@@ -1,7 +1,7 @@
 //
 // Portable drawing function header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2022 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -972,6 +972,9 @@ void fl_draw_check(Fl_Rect bb, Fl_Color col);
 // Draw one or more "arrows" (triangles)
 FL_EXPORT void fl_draw_arrow(Fl_Rect bb, Fl_Arrow_Type t, Fl_Orientation o, Fl_Color color);
 
+// Draw a potentially small, filled circle
+FL_EXPORT void fl_draw_circle(int x0, int y0, int d, Fl_Color color);
+
 // images:
 
 /**