[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.
By the looks COLOR is used only in this class and in the ancient .xpr stuff.

Thus, I suggest you just copy the definition into here (local struct to this class) and use it directly.  Using uint8_t rather than unsigned char so we're being specific about sizing would be good.  The palette information doesn't leave these classes.

I'll prep a PR removing usage of the .xpr stuff.