macOS: Fix "Full screen broken on macOS in FLTK 1.3.10 (regression)" (#1192)

FLTK now supports 2 types of fullscreen modes under macOS:
1) normal macOS fullscreen mode where the menubar and titlebar appear when mouse is moved to top of screen
and the window covers one screen only;
2) multiple-screen mode that requires calling Fl_Window::fullscreen_screens() before.

This commit fixes all possible transitions between these fullscreen modes and normal window mode.