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()
- Add internal fl_contrast_cielab() as the new default.
- Keep old function as internal fl_contrast_legacy().
- Add fl_contrast_mode() to switch between fl_contrast() functions.
- Add fl_contrast_level() to fine tune fl_contrast() per mode.
- Add option to register and use a custom contrast function.
- Add test/contrast.cxx test program.
- Move all fl_contrast() related code to a new file src/fl_contrast.cxx.
- Add fl_lightness() convenience function for perceived lightness.
- Add fl_luminance() convenience function for physical luminance.
