Fix Arceus's mismatched and missing sprites. #189

Its Platinum backsprites followed the veekun database's IDs instead of
the game's internal IDs.  (Our IDs are the same, except for ??? being
moved to the end.)  For example, veekun lists Fire as type #10, but the
gen IV games have ??? as type #10; 493-fire.png was the sprite for ???-
type Arceus.  This is why about half of them were in a loop.

Its Normal-type Platinum backsprites were outright missing, and its ???
Platinum frontsprites were copies of Dark's instead.  I reripped those.

493-normal overworld sprites were missing, but its plain 493.png sprites
were its default, Normal-type form, so I copied those.