'PostMobjThinker' hook

Adds a new hook that runs at the end of P_MobjThinker

In this GIF:

• The blue crawla's sprite is constantly being set to SPR_SPOS (the red crawla's sprite)
• The crawla's translucency is changing without being reset every time it changes frame
• The crawla's explosion graphic to changed to that of the old explosion graphic (using SPR_MINE)

Test Lua: PostMobjThinkerTest.lua