Rename some arcane "cx"/"cy" variables
Better rain drawing fix for shifted viewports

My previous fix for rain in shifted viewports made it so that rain would just not fall after you got too far away from the player. Fixed it the RIGHT way this time.
Actually, really fix rain this time

Bug introduced by the last commit, and fixed by this commit: rain would fall where mobs were standing, replacing them with nothing.

The real issue: there was a variable in map::draw that was being set according to the player character's coordinates, when other code wanted it to be set according to the viewport center. ACTUALLY fix it this time. I hope. Unless there's more magic.
Merge pull request #319 from ethankaminski/drawing-fix

Better fix for rain drawing