[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.
definition is only needed on template instantition. nothing derefs the vector (it's a reference), and nothing derefs a GifFrame (or a FramePtr).  thus instantiation is delayed and you are good.