[WIP] Add proper gif support

The goal of this pr is to add proper gif support using giflib (http://giflib.sourceforge.net/).
Currently only animated texture, e.g., backgrounds, are supported. Help for the picture part is highly welcome ;) Some nice backgrounds can be found at http://www.buzzfeed.com/thegeshow/the-60-most-beautiful-cinemagraph-gifs.

Many thanks to @memphiz for the non win32 part!

The texturepacker gif code seems be erroneous too, that would be the next goal.