[cairo] X crash with BadAlloc with too-large pixmap

[Vladimir Vukicevic]

It seems to be possible to crash the xlib backend by trying to create  
an image with dimensions grater than SHRT_MAX, as would be expected  
due to X's limits.  Mats has a fix for this in bug https://bugzilla.mozilla.org/show_bug.cgi?id=424333 
  -- essentially, it wraps XCreatePixmap to something that checks  
width/height and returns an error in that case.  Does that seem like a  
reasonable fix?

     - Vlad