Polyobject top/bottom texture + translucency

polyobjects do have top/bottom textures, silly.
And translucency.
Polyobject translucency quick fix.

I realise what I did before may cause FOF's in the same sector to not be
drawn, if they were before.
else added

less redundancy.
Fix certain textures with holes in

see: CEZ1 skybox in linear filtermodes.
OpenGL Fade masks

I think I've done this all right, someone correct me if I haven't.
OpenGL: Fix sky drawing

sky positioning and scaling more correct compared to software.
OpenGL: Sky culling

Gets rid of stuff being drawn where the sky should be being drawn.
Quick sky fix

Avoid Z-Fighting on the depth wall with the regular walls to stop the
possibility of regular walls being made partially invisible.
Quick sky fix 2

walls not extending down when a thok barrier has top textures and both
ceilings have sky.
Fix THOK MD2's not rotating with camera

Will also fix any other case that an mobj has a skin and a single frame.
OpenGL: Polyobject Planes
whiteline checkup
Fix hardware / r_opengl warnings.
Merge branch 'master' into next