imlib2: use giflib instead of soon to be deprecated libungif

Add three upstream patch to support giflib version 5.1

Also rename the previous patches to match the order of the commits in
the upstream master branch.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>