OpenGL implementation of all fl_ "Drawing Fast Shapes" graphics calls (#385)

* Fix build system for unites,

* Updated unittest to check OpenGL drawing.

Making sure that OpenGL drawing is exactly the same
as native drawing to make FLTK widget rendering
look the same in GL windows.

* Make OpenGL optional.

* Implemented clipping in OpenGL

* unites drawing fast shapes

* Fixed CMake

* Updating unittest.

Added tests for fl_pi and fl_arc (int)
Renamed tab to render complex shapes.

* Improved OpenGL FLTK drawing emulation.

* Fixed GTK ROUND DOWN BOX

* Fixing Makefile for unittest

* Correctly aligning OpenGL text.

* Fixed text alignment in GL windows.

Explained the "FLTK over GL " example in Cube.

* Overlapping test.

* Better GL graphics alignment.

* Drawing the focus rect.

* Adding Alpha Channel support for GL.

* Added FLTK-on-GL documentation.

FL/Fl.H

@@ -1063,6 +1063,7 @@ int main() {
 
   // color map:
   static void   set_color(Fl_Color, uchar, uchar, uchar);
+  static void   set_color(Fl_Color, uchar, uchar, uchar, uchar);
   /**
     Sets an entry in the fl_color index table. You can set it to any
     8-bit RGB color. The color is not allocated until fl_color(i) is used.
@@ -1070,6 +1071,7 @@ int main() {
   static void   set_color(Fl_Color i, unsigned c); // platform dependent
   static unsigned get_color(Fl_Color i);
   static void   get_color(Fl_Color i, uchar &red, uchar &green, uchar &blue);
+  static void   get_color(Fl_Color i, uchar &red, uchar &green, uchar &blue, uchar &alpha);
   /**
     Frees the specified color from the colormap, if applicable.
     If overlay is non-zero then the color is freed from the

FL/Fl_Gl_Window.H

@@ -67,7 +67,9 @@ class FL_EXPORT Fl_Gl_Window : public Fl_Window {
   int mode(int, const int *);
   static int gl_plugin_linkage();
 protected:
+  void draw_begin();
   virtual void draw();
+  void draw_end();
 
 public:
   void show();