spearmint: Allow refdef_t to drop non-sky surfaces

RTCW SP drops skybox portal ground surfaces if RDF_DRAWSKYBOX is not set.
Add a simpler non-skybox specific RDF_ONLYSKY instead.

Also make RDF_NOSKY/RDF_ONLYSKY include "surfaceParm sky" shaders,
instead of just shaders with skyParms or FAKK's skyportal.

Drop the surfaces before checking dlights, sorting the surfaces, etc
instead of right before rendering.