Add NV2A emulation

[StrikerX3]

Integrate XQEMU's NV2A emulation on the XQEMU adapter (#22).

[StrikerX3]

While at it, make sure to splice OpenGL out of the code. We don't want any hard dependencies on any one graphics API.

[PatrickvL]

Note, that the Cxbx-Reloaded version is a cripled copy of the xqemu nv2a implementation, so I'd advise to largely ignore the former in favor of the latter.

Also, once upon a time, there was an independent nv2a implementation build in xexec by haxar - this might be a valuable resource too for anyone :stuck_out_tongue_winking_eye: that can retrieve it's (now removed from github) code.

[StrikerX3]

Thanks for the info, @PatrickvL. I updated the references above.

[StrikerX3]

JayFoxRox suggested submoduling XQEMU and creating an adapter for its hardware classes.