SL-17194 Impostor quality pass 2 (and fix for fully transparent rigged attachments rendering to depth buffer)

SL-17028 Fix for inconsistent ordering of transparent rigged meshes and removal of glow-only rigged meshes from depth buffer.

Due to changes in SL-16715

Related to fix for SL-16792, standardize on naming the filed simply mfa_hash end-to-end to avoid confusion

+ post merge conflict fix

SL-16468 Fix for crash when enabling highlight transparent (add rigged mesh support to highlight transparent).

SL-16202 Put Multi-threaded GL behind a feature flag and update featuretable (decruftify settings, compatibility pass).

Reverting a merge is sticky: it tells git you never want to see that branch
again. Merging the DRTVWR-546 branch, which contained the revert, into the
glthread branch undid much of the development work on that branch. To restore
it we must revert the revert.

This reverts commit 029b41c0.

This reverts commit 5188a26a, reversing
changes made to 81908856.

SL-16148 SL-16244 SL-16270 SL-16253 Remove most BlockTimers, remove LLMemTracked, introduce alignas, hook most/all reamining allocs, disable synchronous occlusion, and convert frequently accessed LLSingletons to LLSimpleton

ThreadPool bundles a WorkQueue with the specified number of worker threads to
service it. Each ThreadPool has a name that can be used to locate its
WorkQueue.

Each worker thread calls WorkQueue::runUntilClose().

ThreadPool listens on the "LLApp" LLEventPump for shutdown notification. On
receiving that, it closes its WorkQueue and then join()s each of its worker
threads for orderly shutdown.

Add a settings.xml entry "ThreadPoolSizes", the first LLSD-valued settings
entry to expect a map: pool name->size. The expectation is that usually code
instantiating a particular ThreadPool will have a default size in mind, but it
should check "ThreadPoolSizes" for a user override.

Make idle_startup()'s STATE_SEED_CAP_GRANTED state instantiate a "General"
ThreadPool. This is function-static for lazy initialization.

Eliminate LLMainLoopRepeater, which is completely unreferenced. Any potential
future use cases are better addressed by posting to the main loop's WorkQueue.

Eliminate llappviewer.cpp's private LLDeferredTaskList class, which
implemented LLAppViewer::addOnIdleCallback(). Make addOnIdleCallback() post
work to the main loop's WorkQueue instead.

Make LLAppViewer::idle() call LL::WorkQueue::runFor() to dequeue and run some
or all of the pending responses from worker threads.

Add a MainWorkTime setting to specify the time slice the main loop may devote
each frame to servicing such responses.

SL-16102 Set window title to agent name (child of SL-15999 Support for low overhead, non interactive viewer sessions)

SL-16043 Fix DisableVerticalSync, default DisableVerticalSync to off, remove broken frame limiter code (use vsync if you want to limit framerate).

This reverts commits:
126877bc
b661f62a
ce660c7e