[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.
You probably want to check m_lasttime = 0 here.  If so, update m_lasttime but don't update the frames etc.