[RP] frestore and multi-head

[Trent Buck]

I've worked out why :frestore crashes sometimes when using multi-head
(xinerama or multiscreen) displays.

It happens when you try to restore a frame with the same number as a
frame on a different screen.

To reproduce:

  :fdump ==> 0 0 0 800 600 0 0,
  :nextscreen
  :fdump ==> 1 0 0 800 600 0 2,

  Frame 0 is in use on screen 0.  Try to restore a `frame 0' while still
  on screen 1.

  :frestore 0 0 0 800 600 0 2, ==> segfault

I'm still grokking cmd_frestore, so no fix yet.

-trent
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