Resize the framebuffer when switching modes with VidMode. This allows better interaction with randr, which may have shrunk the framebuffer when changing to a smaller mode.
I'm not totally sure this is the right thing to do and it currently segfaults on the nVidia driver because the functions are never filled in by
xf86CrtcConfigInit() there. But without this, we can't get games that use VidMode to fullscreen after changing modes via RandR.