[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.
yes i really wondered - but believe me - if i get rid of that pointer it complains about incomplete type "COLOR" :)