Remove global state

Using global state (`ShInitialize`, `InitGlobalLock`, `PerProcessGPA` etc.) is bad API design and makes using glslang in multi-threaded applications both risky and difficult.

You should group this global state into a “context” struct and pass it around explicitly where needed.

Start by tracing the named global-state entry points and symbols: ShInitialize, InitGlobalLock, and PerProcessGPA. Map where that state is created and consumed before deciding how a context struct could be passed through the relevant paths. Done means global state is replaced by explicit context passing and multi-threaded use is no longer risky.

Written by the indexing model from the issue text.