This fixes creating apng files, which is broken since commit
5ef19590.
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 940b8908)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>