Currently Windows gets this from vcpkg. Mac will get it from Homebrew,
and the GitHub linux builders will just install it with apt-get.
This might not be the perfect solution (having to install Homebrew is a pain,
GitHub Actions has an ancient SDL2), but it gets all the common platforms
running for now without much fuss.
This is all in unused/unnecessary/leftover code, but the code wouldn't
build without Xlib headers on the system, which was forcing the Mac builds
to install xquartz.
This also removes renderer/SoftwareOpenGL*, which was _not_ a
software-rendered OpenGL, but just an older version of the same code that's
in HardwareOpenGL.cpp--old enough to still talk to glX directly, which is
what caught my attention.
