implemented fake reflection cubemaps... requires that a .shader

implemented fake reflection cubemaps...  requires that a .shader
specifies dpreflectcube with a cubemap prefix, and requires that the
material textures include a _reflect.tga image to mask the reflection
(roughly similar to _gloss texture)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9871 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=8d0aa212a91a456c5f7cc13963cc9f501e122ee4