They take a pointer to a free-list used for hashlinks so the hashlink
pools can be per-thread. However, hash tables that are not updated are
always thread-safe, so this affects only updates. progs_t has been set
up such that it is easy for multiple progs within one thread can share
hashlinks.
Again, based on The OpenGL Shader Wrangler. The wrangling part is not used
yet, but the shader compiler has been modified to take the built up shader.
Just to keep things compiling, a wrapper has been temporarily created.
