playermodel showcase Update 2

Update 2 adds the following to the map:
A new Halloween room
All Halloween Bosses with their skins
Zombie Mercenaries
Redmond and Blutarch Mann
Uncommon Payloads (EOTL and Dirty Bomb payloads)
The MvM Tank models are now in the 2nd room
Added a sentry buster to the bot lineup

Update 1: Added author credits, snow for the decal tester, and LDR cubemaps