fix ofAppGLFWWindow fullscreen on multimonitor systems

# issue

Contrary to ofAppGLUTWindow, ofAppGLFWWindow would always only go fullscreen on the *primary* monitor, regardless on which monitor the window was placed before going fullscreen.

This commit fixes this by replicating the behaviour of ofAppGLUTWindow, following the maxim of least surprise.

+ in multi-monitor scenarios, windows will go fullscreen on the monitor they inhabited before they switched into fullscreen mode.

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>