V2.x open gl3.0 compat

There's a lot about the GZDoom source I don't know, so how I fix some of these issues might not be the best way to fix them. The initialization of gl.maxuniformblock in particular I'm not sure about. One the one hand, a printing function probably shouldn't also be initializing a global, but again I'm not sure how old that mingw comment is. It might not crash now, so removing the ifdef might be good enough. As it is, the various GL info lines aren't printed to the console on linux.

Also, while reducing the GL requirement works now, I have no idea if you have plans that WILL require it.

I did test this using both doom2.wad/MAP01 and tutnt-v108.pk3/TNT01. I didn't notice anything wrong.

If you want me to fix something differently, just let me know and I'll do it.