OpenGL Sprite Billboarding

Should work on both normal and precipitation sprites. Sprites that are split by lighting should work correctly. (Papersprite parts that stopped it compiling were removed from this commit, I'll create an MR for the papersprites branch if needed).

Main topic of discussion for this is probably whether a cvar is desired. Might be more necessary for vanilla since looking up/down by will is a thing. A cvar has been added.