Merge branch 'private_flatsprite' into 'master'

OpenGL papersprites

I don't know if anyone actually tested Sryder's original OpenGL papersprites support before it was merged in, but there was some issues with the papersprites themselves "wobbling" about in a sort of weird way depending on the angle you view it from or where you are. It's easiest to see what I mean if you just load up sawb.wad, and strafe about when viewing one of the sawblades.

Thankfully all I really needed to do was tweak the way angles work for papersprites. I also cleaned up Sryder's original code a little, not that I really needed to do so though.

See merge request !96