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.

1 files changed, 87 insertions, 0 deletions

diff --git a/test/unittests.h b/test/unittests.h
new file mode 100644
index 000000000..3b9cddd3f
--- /dev/null
+++ b/test/unittests.h
@@ -0,0 +1,87 @@
+//
+// Unit tests for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 1998-2022 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
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+#ifndef UNITTESTS_H
+#define UNITTESTS_H 1
+
+#include <FL/Fl.H>
+#include <FL/Fl_Double_Window.H>
+
+// WINDOW/WIDGET SIZES
+#define MAINWIN_W       700                             // main window w()
+#define MAINWIN_H       400                             // main window h()
+#define BROWSER_X       10                              // browser x()
+#define BROWSER_Y       25                              // browser y()
+#define BROWSER_W       150                             // browser w()
+#define BROWSER_H       MAINWIN_H-35                    // browser h()
+#define TESTAREA_X      (BROWSER_W + 20)                // test area x()
+#define TESTAREA_Y      25                              // test area y()
+#define TESTAREA_W      (MAINWIN_W - BROWSER_W - 30)    // test area w()
+#define TESTAREA_H      BROWSER_H                       // test area h()
+
+typedef void (*UnitTestCallback)(const char*, class Fl_Group*);
+
+extern class MainWindow *mainwin;
+extern class Fl_Hold_Browser *browser;
+
+enum {
+  kTestAbout = 0,
+  kTestPoints,
+  kTestFastShapes,
+  kTestCircles,
+//  kTestComplexShapes,
+  kTestText,
+  kTestSymbol,
+  kTestImages,
+  kTestViewport,
+  kTestScrollbarsize,
+  kTestSchemes,
+  kTestSimpleTerminal
+};
+
+// This class helps to automatically register a new test with the unittest app.
+// Please see the examples on how this is used.
+class UnitTest {
+public:
+  UnitTest(int index, const char *label, Fl_Widget* (*create)());
+  ~UnitTest();
+  const char *label();
+  void create();
+  void show();
+  void hide();
+  static int numTest() { return nTest; }
+  static UnitTest *test(int i) { return fTest[i]; }
+private:
+  char *fLabel;
+  Fl_Widget *(*fCreate)();
+  Fl_Widget *fWidget;
+  static void add(int index, UnitTest *t);
+  static int nTest;
+  static UnitTest *fTest[];
+};
+
+// The main window needs an additional drawing feature in order to support
+// the viewport alignment test.
+class MainWindow : public Fl_Double_Window {
+public:
+  MainWindow(int w, int h, const char *l=0L);
+  void drawAlignmentIndicators();
+  void draw();
+  void testAlignment(int v);
+  int fTestAlignment;
+};
+
+#endif