[SP] Merge some g_client cleanup/rewrite stuff to SP from MP.
Removed _NO_PCH and _IMMERSION references.

_NO_PCH will always be true, and _IMMERSION references nothing, so out
they go.
Removed unused files.
[SP] Removed g_headers files.

Files now explicitly depend on the headers they need, instead of that
catch all.
Fix build break.
Adding myself as a maintainer.
Updated JK2 readme.
Make the readme more readable.
[SP] More renderer cleanup. Remove unused files (null, .def, .zip)
[SP] Fix bad merge referencing moved file