Use the invariant keyword on gl_Position on GLSL 1.20 or higher and GLSL ES

Use the invariant keyword on gl_Position on GLSL 1.20 or higher and GLSL ES
1.00 or higher, this should resolve zfighting between light polygons and
base surfaces.

A long long time ago this was using ftransform() which ensured invariance but use of the invariant keyword was not added when we switched away from that.

Thanks for the bug report:
https://www.reddit.com/r/quake/comments/6rwwm5/need_some_help_getting_quake_running_with/

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12336 d7cf8633-e32d-0410-b094-e92efae38249