DRTVWR-559 Revert skies to be very close to release and disable tone mapping when probe ambiance is zero.

Hack for desaturating legacy materials has been removed for performance and quality reasons.

Adds a new setting for auto adjusting legacy skies.  This is the PBR "opt out" button.  If disabled, legacy skies will disable tonemapping, automatic probe ambiance, and HDR/exposure.  If enabled, legacy skies will behave as if probe ambiance and HDR scale are 1.0, and ambient will be cut in half.  

HDR scale will act as a sky brightener, but will automatically adjust dynamic exposure so the sky will be properly exposed.  If you want relatively even exposure all the time, set HDR Scale to 1.0.  If you want a high range of exposures between indoor/dark areas and outdoor/bright areas, increase HDR Scale.

Also tuned up SSAO (thanks Rye!).

Reviewed with Brad.

SL-19713 Fix for broken avatar preview render in animation upload.  Incidental decruft and camera FoV network spam reduction.

* Fix water fog mismatch between alpha blend and opaque when local light is nearby

* Add PBR underwater alpha blend shader

DRTVWR-559 Remove RenderReflectionProbeCount (which is bugged) and lean on RenderReflectionProbeLevel for preferences (which works).

DRTVWR-583
DRTVWR-559

DRTVWR-583

* Add a binary cache for compiled shaders using glProgramBinary

* Add additional sanity checking to shader binary save and load, hook up cache clear and menu option

* Fix default init of shader cache data struct and clear gl errors before glGetError calls

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Co-authored-by: RunitaiLinden <davep@lindenlab.com>

SL-19655 Remove concept of "low priority" updates (RenderDebugGLSession assertion stomping pass).  Remove a couple unused debug settings and related code.

following promotion of DRTVWR-577

SL-19674 Disable "Blinn-Phong" when a PBR material is applied to make it clear that those buttons are inoperable until you remove the PBR material.

SL-19709 Fix for fullbright shiny not factoring out exposure and flickering fullbright alpha.  Incidental decruft.

SL-19710 Fix for deadlock in LLPerfStats.  Move General thread initialization to earlier in session (unrelated).

DRTVWR-583

SL-19656 Remove LLPerfStats thread and fold into General thread.  Hook avatar GPU time into LLPerfStats.  Incidental decruft.