Changes:

• Make dimmer more nvidia driver friendly, fixing a crash on large multi-monitor configurations by limiting the complexity of render calls; controlled using XSECURELOCK_DIM_MAX_FILL_SIZE.
• Add a weaker force-grab method that merely steals input focus; XSECURELOCK_FORCE_GRAB still controls the stronger method, but it should trigger less often now.
• Fix crash with colored emoji fonts (still can't render emoji; we're now simply rejecting fonts Xft cannot handle).
• Also run the switch user dialog on Ctrl-Alt-Zero as an alternative to Ctrl-Alt-Oh; more user friendly as in some fonts, Ctrl-Alt-O looks like both.
• XFlush when doing things affecting X11 idle timers.