costumizable canister color

http://i.imgur.com/GAmAQMP.png

Allows for 4 costum layers of canister labelling, so now you can be more specific about canister defenitions than ever before!

you can also add decals now, so even MORE canister costumization!
they're easy to add too, you need to add them to 1 list in the code and make a sprite for them.
The PR comes with 3 decals: the little P on plasma canisters, because i't look bad if it was part of the plasma texture, and 2 (VERY ugly sprited) decals that are a nice way to show if a canister's supposed to be hot(toxins burn mixes) or cold(cryogenics, SM autism)
potentially dangerous: might want to replace with ```while(src && !src.gc_destroyed && usr)```