- 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

- New option `-a`: Play animations at startup
- Ctrl-Space toggles animation for all GIF files
- Infinite loop for all animations

Fixes issue #143

- Scale mode is not reset to default value upon image loading anymore
- New default key binding to change mode to scale-down
- Removed scale mode setting from config.h
- Removed -d command line option, as this is now the default at startup

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.

This reverts commit f795273b65.

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.