Two colors are more than enough!

Related to issue #303

Before that zoomdiff was a merely heuristic.

Related to issue #303

Fixes issue #302

Fixes issue #295

The imlib2 TGA loader returns an imlib image handle without any actual data
when given a text file like this:

    T
    Content-Type: application/javascript
    Content-Length: 3836
    Last-Modified: Wed, 23 Sep 2015 12:25:47 GMT
    Etag: "56029a4b-efc"
    Expires: Sat, 20 Aug 2016 15:14:33 GMT
    Cache-Control: max-age=604800, public
    Accept-Ranges: bytes

Fortunately, `imlib_image_get_data()` returns NULL in this case, so that we can
use it as an additional check when opening files.

This makes mouse panning more direct and faster.

second while maintaining backward compatibiitiy with integer
arguments.

- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)

- 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

- 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

This reverts commit f795273b65.

Also removed now obsolete -p command line option; fixes issue #98