[RetroDays] Composing is slower than NeoDays

[Golfavel]

Composing RetroDays is consistently much slower (~3x-5x) than NeoDays, despite having less sprites and less colors (mostly due to transparency).
I don't know the exact reason behind it, however looking at the sprites between the two, I believe the slowdown has to do with the extra size that RetroDays holds, which itself seems to be from GIMP's default metadata inclusions when exporting. If such is the case, the solution should be fixable by trimming this excess data (e.g., similar to what was done in #1647 on 20x20 sprites).

Testing
./poser.sh is a bash script used to automate some additional steps in testing spritesheets and changes. It can be replaced with the composer.py tool provided in the repository for reproduction

john@vm:~/Documents$ echo "retrodays:" && time ./poser.sh Retrodays RetroDaysTileset && echo "neodays:" && time ./poser.sh NeoDays NeoDaysTileset
retrodays:
    
real    0m24.090s
user    0m23.018s
sys     0m1.305s
neodays:

real    0m8.331s
user    0m6.377s
sys     0m1.670s
john@vm:~/Documents$ echo "retrodays:" && time ./poser.sh Retrodays RetroDaysTileset && echo "neodays:" && time ./poser.sh NeoDays NeoDaysTileset
retrodays:

real    0m26.059s
user    0m25.021s
sys     0m1.539s
neodays:

real    0m6.477s
user    0m5.884s
sys     0m1.093s
john@vm:~/Documents$ echo "retrodays:" && time ./poser.sh Retrodays RetroDaysTileset && echo "neodays:" && time ./poser.sh NeoDays NeoDaysTileset
retrodays:

real    0m23.595s
user    0m23.173s
sys     0m1.194s
neodays:

real    0m5.856s
user    0m5.661s
sys     0m0.905s
john@vm:~/Documents$ echo "retrodays:" && time ./poser.sh Retrodays RetroDaysTileset && echo "neodays:" && time ./poser.sh NeoDays NeoDaysTileset
retrodays:

real    0m25.244s
user    0m25.002s
sys     0m1.230s
neodays:

real    0m6.344s
user    0m5.974s
sys     0m1.171s
john@vm:~/Documents$ echo "retrodays:" && time ./poser.sh Retrodays RetroDaysTileset && echo "neodays:" && time ./poser.sh NeoDays NeoDaysTileset
retrodays:

real    0m23.062s
user    0m22.942s
sys     0m1.099s
neodays:

real    0m5.711s
user    0m5.491s
sys     0m0.982s