Fixed SF bug #1204256: Off-by-one error in X11 mode switching.

Fixed SF bug #1204256: Off-by-one error in X11 mode switching.
The code which changes the X11 video mode has an off-by-one error,
resulting in a wrong mode or segfault. I've refactored the code
slighty to make it hopefully clearer what's going on and corrected
some #ifdefs on the way. Now at least the "One" demo works for me.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@615 7f0cb862-5218-0410-a997-914c9d46530a