Graphical tweaks and fixes, also a background graphic for Agility Save

- The credits weren't actually translucent because Add doesn't work the same in
  TEXTURES as it does in actual sprite rendering. Whoops.
- Fixed the MUN3-MAG to better resemble the fixed credits.
- Added a background graphic for Agility Save - I had created it but had
  forgotten to actually use it anywhere.
Fixed some stuff related to the stored monster positions and the Hell Unleashed event
Yeah, let's put this in
Attempting to fix mutliplayer monster death mission handling, currently still broken
Fixed now
Merge branch 'master' of https://github.com/Kyle873/DoomRPG into drlaupdate