- Buffer for window content is bigger than the window, minimizes artifacts
  when window is resized
- Back to using XSetWindowBackgroundPixmap() instead of XCopyArea(),
  no need to handle exposure events; X server can show gray background directly
  after resize event before sxiv redraws the window contents

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

Issue described here:
https://bbs.archlinux.org/viewtopic.php?pid=1117294#p1117294

Gets called on all unset key mappings. Arguments are: key combo and
current file. Thanks to Francesco Orsenigo (xarvh) for the idea.

- Command it_toggle_image_mark (bound to 'm') toggles mark of current
  image
- Command it_navigate_marked (bound to 'N'/'P') can be used to go to
  the next/previous marked image
- When option -o is given, all marked files get printed

Reported by Kris Siwiec

Fixes issue #84

Without this some window managers (e.g. fvwm) will ignore the initial
window position and place it according to it's own rules.

- Draw onto pixmap as before, but use the same size for the pixmap as
  for the window, allocate new pixmap after configure requests
- Use XCopyArea() instead of XSetWindowBackgroundPixmap(), which now
  requires handling of Expose events