Add csqc_polygons_defaultmaterial_nocullface cvar which defaults to 1 in nexuiz and...

Add csqc_polygons_defaultmaterial_nocullface cvar which defaults to 1 in nexuiz and xonotic derived games, this restores the compatible behavior with older versions of the engine.  It defaults to 0 in other games (and would be recommended to set to 0 in those games when they make a new release) as it seems like that would be the better default for efficiency (and matches FTEQW engine).

Fixed a bug in CL_MeshEntities_FinalizeRenderEntity where it skipped finalize when the entity has no surfaces, but since it is still included in the render entities list it was rendering garbage when unused.

Use the "$whiteimage" texture for CSQC/UI polygons (VM_CL_R_PolygonBegin) that specify texture "", to be compatible with older versions.

Clarify the name of a parameter on Mod_Mesh_GetTexture prototype to match the implementation as it was misleading otherwise.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12569 d7cf8633-e32d-0410-b094-e92efae38249