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.
The vast majority of this is fixing up `char *` that should be `const char *`
but a handful of other fixes, like potential buffer overflows that GCC
noticed, etc, were applied as well.
This removes `-Wno-write-strings` from CMakeLists.txt, as it is no longer
necessary, as there is no longer a flood of compiler warning spam when
building.
This does not fix all compiler warnings; there are still a handful, and they
are legitimate, but they can be dealt with in a future commit.
