- 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)

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

This reverts commit f795273b65.

- Corrected handling of transparency settings for individual frames
- Corrected handling of different frame dimensions & offsets

This reverts commit 090ee5405b.