We now use dlsym() to check for XMesaSetFXmode() at runtime.
If we find XMesaSetFXmode() all DGA code is disabled so it won't clash
with Glide (Glide on Voodoo 3 uses DGA internally...)
For XMESA we now check the MESA_GLX_FX environment variable at startup to
detect whether we are started in windowed or fullscreen mode, and then
set the cvar vid_glx_fullscreen to match that.