SL-16202: Use WorkQueue::postTo() for texture create/post handshake.

That is, when LLViewerFetchedTexture::scheduleCreateTexture() wants to call
createTexture() on the LLImageGLThread, but postCreateTexture() on the main
thread, use the "mainloop" WorkQueue to set up the handshake.

Give ThreadPool a public virtual run() method so a subclass can override with
desired behavior. This necessitates a virtual destructor. Add accessors for
embedded WorkQueue (for post calls), ThreadPool name and width (in threads).

Allow LLSimpleton::createInstance() to forward arguments to the subject
constructor.

Make LLImageGLThread an LLSimpleton - that abstraction didn't yet exist at the
time LLImageGLThread was coded. Also derive from ThreadPool rather than
LLThread. Make it a single-thread "pool" with a very large queue capacity.