New shader keyword dptransparentsort (can be "sky", "distance", "hud") which forces...

New shader keyword dptransparentsort (can be "sky", "distance", "hud") which forces one of transparent sort techniques. EF_NODEPTHTEST and RENDER_WORLDOBJECT entity flags have greater priority and override shader-set transparent sort technique.
New cvar q3shader_force_terrain_alphaflag which sets TEXF_ALPHA on both vertextextureblend textures (fixes r_glsl_vertextextureblend_usebothalphas).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11822 d7cf8633-e32d-0410-b094-e92efae38249
::stable-branch::merge=2048a824c4d5c5d4d031e519dd3a125db35415fd