Do not build VBOs on a dedicated server.

Do not build VBOs on a dedicated server.

Fixes crash at startup of dedicated server in Quake and Xonotic
(probably in everything).

The only reason why this ever worked before was that Mod_BuildVBOs's
main callee R_Mesh_CreateMeshBuffer did a GL2 feature test before
calling into anything VBO related (and on a dedicated server, which does
not initialize GL, the test always returns false); however now that we
require GL32, the feature test is gone and thus the crash happened. An
explicit check for being a dedicated server fixes that.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12469 d7cf8633-e32d-0410-b094-e92efae38249