After all these tests, I conclude that GDI+ is not convenient for text drawing

and is not useful for drawing horizontal and vertical lines.

New proposal for fellow developers to evaluate:

use GDI+ only when it allows antialiasing beyond what GDI can do.

The modified code is in another branch I called    "GDI+soft"

   git clone https://github.com/ManoloFLTK/fltk.git

   git checkout GDI+soft

   autoconf -f

   ./configure                 <----- not necessary to activate any special option here

   make

CMake-based build works also. Default settings are enough.

GDI+ is initialized when the app starts and is shutdown when it stops.

GDI+ is called at each call of fl_line_style() to change accordingly the GDI+ pen.

GDI+ is called by these drawing functions:

fl_arc(int, int, …)

fl_pie()

fl_line()

fl_loop()

fl_polygon()

fl_circle()

and all functions to draw "complex shapes".
All the rest (text, horizontal and vertical lines, rectangles, boxes, images) is unchanged: GDI is used.

App test/unittests evidences the benefit ot antialiased circles, ellipses and oblique lines,

and test/device that with clock hands and complex shapes.

The plastic and gleam schemes are completely GDI-based, so there's no slowdown

when they are used.

Configure.ac checks for presence of GDI+ and  doesn't compile GDI+-using code in its

absence. Thus, the FLTK library remains compatible with Windows95.