Workaround options window appearing behind viewer

Workaround options window appearing behind viewer

FLTK's fullscreen_x() function will always put the window on a high
level (NSStatusWindowLevel = 25), even if the window doesn't have focus.
This causes the OptionsDialog to end up behind the DesktopWindow when
the fullscreen function is called. Until we can rest assured that most
people building TigerVNC will use a fixed version of FLTK, we will need
this workaround.

(cherry picked from commit 9023a2ec1823d473f1cfe0b723c8636d8e994e2c)