Invert OpenGL rollangle for papersprites to match software renderer behaviour

Resolves #849 (closed). I decided to invert OpenGL's rollangle behaviour rather than software's, because I think it's more intuitive that a positive rollangle points the paper sprite upwards relative to its facing angle.

Here is a test script that turns all players into papersprites and adds a rollangle console variable that lets you update their rollangle variables. Players should look consistent in both renderers. paper.lua