Further work on RPG

- Sprites are now functional outside of on_hand sprites which for some
reason still show the default gun skin
- Functionality is about the same, but a nasty bug has been witnessed
with spawned RPGs in some cases, handle with precaution
- RPGs will be added to the map once I manage to stop the gun from
dropping casings and make sure the gun itself doesn't have serious
glitches
More additions to RPG

- RPG now has a back sprite, it hasn't been tested but has been resized
and positionned in a fashion alike the spear
- RPG now starts unloaded, this should fix all problems related to
incorrect spawning
Adding empty RPG back sprites

- Doesn't actually show as of current, but asset is here
Merge pull request #1206 from dylanstrategie/Rocket

Further work on RPG