changed the X11 gamemode code policy. Falling back to xf86vm when xrandr fails

changed the X11 gamemode code policy. Falling back to xf86vm when xrandr fails
was problematic when passing invalid modes. Now instead, if xrandr is
available, we use that, and only that. XF86VM is now only used when XR&R is not
supported at all.

I also added a set of env vars to artificially disable one or the other, or
both. If FREEGLUT_NO_XRANDR is set we completely ignore XR&R, and if
FREEGLUT_NO_XF86VM is set, we completely ignore XF86VidMode. Obviously if both
are defined or missing, game mode can't switch resolutions at all any more, but
that's a nice debugging aid, and may be useful to allow the user to pick which
extension he'd rather use for mode switching.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1695 7f0cb862-5218-0410-a997-914c9d46530a