This reverts commit 2f964c0a.

The previous implementation went to some effort to crash if anyone attempted
to create or destroy an LLInstanceTracker subclass instance during traversal.
That restriction is manageable within a single thread, but becomes unworkable
if it's possible that a given subclass might be used on more than one thread.

Remove LLInstanceTracker::instance_iter, beginInstances(), endInstances(),
also key_iter, beginKeys() and endKeys(). Instead, introduce key_snapshot()
and instance_snapshot(), the only means of iterating over LLInstanceTracker
instances. (These are intended to resemble functions, but in fact the current
implementation simply presents the classes.) Iterating over a captured
snapshot defends against container modifications during traversal. The term
'snapshot' reminds the coder that a new instance created during traversal will
not be considered. To defend against instance deletion during traversal, a
snapshot stores std::weak_ptrs which it lazily dereferences, skipping on the
fly any that have expired.

Dereferencing instance_snapshot::iterator gets you a reference rather than a
pointer. Because some use cases want to delete all existing instances, add an
instance_snapshot::deleteAll() method that extracts the pointer. Those cases
used to require explicitly copying instance pointers into a separate
container; instance_snapshot() now takes care of that. It remains the caller's
responsibility to ensure that all instances of that LLInstanceTracker subclass
were allocated on the heap.

Replace unkeyed static LLInstanceTracker::getInstance(T*) -- which returned
nullptr if that instance had been destroyed -- with new getWeak() method
returning std::weak_ptr<T>. Caller must detect expiration of that weak_ptr.

Adjust tests accordingly.

Use of std::weak_ptr to detect expired instances requires engaging
std::shared_ptr in the constructor. We now store shared_ptrs in the static
containers (std::map for keyed, std::set for unkeyed).

Make LLInstanceTrackerBase a template parameterized on the type of the static
data it manages. For that reason, hoist static data class declarations out of
the class definitions to an LLInstanceTrackerStuff namespace.

Remove the static atomic sIterationNestDepth and its methods incrementDepth(),
decrementDepth() and getDepth(), since they were used only to forbid creation
and destruction during traversal.

Add a std::mutex to static data. Introduce an internal LockStatic class that
locks the mutex while providing a pointer to static data, making that the only
way to access the static data.

The LLINSTANCETRACKER_DTOR_NOEXCEPT macro goes away because we no longer
expect ~LLInstanceTracker() to throw an exception in test programs.
That affects LLTrace::StatBase as well as LLInstanceTracker itself.

Adapt consumers to the new LLInstanceTracker API.

Remove binding and use of glIsShader for now (only useful for avoiding warnings about deleting shader objects re-used across programs).

Make vograss objects set their face vert/index counts to 0 when setting
the number of blades to 0 to disable rendering so we don't allocate space
and then skip setting it up with actual index data in getGeometry.

Add code to force a usage hint when VBOs are disabled and make useBVOs reflect that as well.

Mods to make merge with viewer-release have less whitespace-only change and fix diffs between EEP and VR.

Fix broken matrix management code causing modelview mats to be used as tex anim mats
(do not depend on default matrix mode already being set).

Give the deferred terrain shader a name.

Make LLRender use the eMatrixMode enum instead of ints.

Fix distortion map rendering in deferred mode not including underwater fog effects.

Fix distortion map rendering not including post-deferred content at all.

Fix distortion map rendering not including anything but sky when camera is underwater.

Update sun_up_factor/sunmoon_glow_factor uniforms even when sun disc isn't in use.

SL-9973

MAINT-7548 MAINT-7560 use NvidiaOptimusEnablement globally exported var to signal desire to use high-performance graphics processor

libatmosphere integrated in indra/llrender/llatmosphere.cpp

Still working on runtime shaders to use libatmosphere precomputed atmospherics textures

These changes are only enabled if RenderNsightDebugSupport is true and eliminate use of
some OpenGL legacy functionality which is incompatible with nSight capture
(mostly glReadPixels and other fixed-function pipe rendering calls).

MAINT-6913 - (Via Sovereign Engineer and Shyotl Kuhr) Pack the skinned matrix and translation into a single mat3x4 for optimal data transfer to reduce uniform slot usage.