Bad x11 window icon code

The current X11 icon code both sets the icon_pixmap and icon_mask in the
WMHints, as well as an icon_window with the comment

/* This is necessary for working properly with Enlightenment, etc. */

Now I wonder exactly why this was necessary, as it breaks icons on other
window managers (fvwm for example). If someone running "Enlightenment, etc"
can try and see if removing that #define USE_ICON_WINDOW in SDL_x11wm.c
makes any difference (and in what way), I’d be happy to hear it.

Otherwise, I’ll disable the icon window code in my next icon patch,
to keep other WM’s working. (thanks to Jonathan Atkins for reporting the bug)