[gif] - add gif support via giflib

This is a split off from #5166

This removes the old animatedgif impl. I was not able to test if this introduces regressions in any way. I just don't know how to trigger that old animatedgif code to verify that it still works.

We use giflib5 for depends and windows - but the implementation also supports the giflib4 api.
here my proposal for fixing that one:

https://github.com/Memphiz/xbmc/commit/5e9387c5f1556e50ecf52a70d3aff7b9504ec5df

since we want to move away from boost i don't want to use any more dependencies to it ...