FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()
* 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.
