[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.
ok this stuff is really @ace20022 's thing ... (i only pull this stuff through the squash/rebase work - but never looked into it...)