Use stencil test to not draw floors where there are chasms

src/Render.hs

@@ -153,6 +153,7 @@ renderShadows shadrendertype nWalls nSils nCaps positiontexture normaltexture li
     -- 1. stencil out the shadows from this light's point of view
     -- 2. calculate lighting based on each fragment's position and normal
     flip VFSM.mapM_ (VFSM.fromList lightPoints) $ \(V3 x y z, rad, V3 r g b) -> do
+        -- note that the stencil shadows rely on the depth buffer
         glDepthFunc GL_LESS
         -- setup stencil
         glColorMask GL_FALSE GL_FALSE GL_FALSE GL_FALSE