implemented uint16 element array support, and use of it for all the

implemented uint16 element array support, and use of it for all the
engine-generated geometry (particles, skybox, text, etc), as well as for
any map or model that fits the 65536 vertex limit (this can be disabled
for performance testing using the cvar gl_mesh_prefer_short_elements),
this hopefully improves performance on Radeon 9500-X300 cards and
GeForce1/2 cards which don't support uint32 elements

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8252 d7cf8633-e32d-0410-b094-e92efae38249