Fix six image skybox having a black border around the sides of the sky
when using OpenGL 1.1 (using CL_CLAMP instead of GL_CLAMP_TO_EDGE).
It's technically visible in q3dm10 but it's more obvious in Team Arena
maps such as mpteam6.
Explicitly set cull type for skybox to front, instead of using whatever
cull type the previous shader used (which could result in the skybox
not being visible due to only drawing back faces). The sky cloud stages
set the cull type so they are not affected by previous cull type.
