Merge tag 'v1.6.0' into debian

Release version 1.6.0.

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.