Sky Rework

[dashodanger]

The current method (assuming Simple Skies are Off) of writing "sky planes" and "sky walls" to the depth buffer and then drawing the sky while juggling depth testing is a bit convoluted and doesn't mesh well with the Sokol rendering path.

Ideally the sky would simply be drawn once before any other rendering or BSP traversal occurs and without needing to determine whether or not the sky "should" be drawn in a frame. The downside of this is that we would still need a way to determine sky planes and walls for the purposes of not drawing anything "behind" them.

[pbdot]

I believe GZDoom is using the stencil buffer for portals, that are generated while traversing and hit sky walls/planes. This would probably simplify this quite a bit. We would have to add support for stencil commands into the customized sokol gl, which think would be pretty easy.

https://github.com/ZDoom/gzdoom/blob/master/src/rendering/hwrenderer/scene/hw_portal.cpp