Change scene transition fade in/fade out to use `Replace` instead of `PopMultiple`

[joetsoi]

ggez/ggez-goodies#16

I originally branched ggez-goodies to push and pop multiple scenes, but it's easier to stay in line with the ggez-goodies rather than maintaining a fork, so switching the fade in/fade out transitions to use Replace as suggest by icefoxen in the linked issue allow us to stop using a fork

[Bhaal22]

I can have a look