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-18190 Cleanup -- convert some vec4's to vec3's (as they really are), remove some unused glsl files.

SL-18239 Unify PBR and non-PBR treatment of ambient/SSAO/irradiance.  Restore SSAO to release version.

SL-18190 WIP -- Take 2 on linear space windlight (more methodical approach -- make desired interface but brute force color conversions). Placeholder PBR water and move to deprecate forward rendering shaders.

SL-14113: Remove magic numbers. Take advantage of existing shader #defines injection by extending shader loading to make use of addConstant() instead of hard-coding magic number constants in each GLSL file.

SL-14113: Remove magic numbers. Take advantage of existing shader #defines injection by extending shader loading to make use of addConstant() instead of hard-coding magic number constants in each GLSL file.

SL-14731: Fix sun gamma -- merge Sovereign Engineer's patch: Fix sun gamma space for accurate color reproduction of sun textures

Removed some potential div-by-0 NaNs and a mangled clamp.

Had to find a balance in the sunlight intensity calculation that behaves correctly for
both these issues.

WIP - Windlight sun lighting should happen in sRGB space, not linear space.  This keeps ambient from getting overblown and better matches environment lighting with ALM on/off.

The un-water-fogged gap area was rendering while we were still above water, but the water surface was being near culled.

I've re-enabled preculling water objects (which also fixes issues with water culling vs spinning the camera) and it seems to behave correctly.

Remove all references to unused shader var global_gamma.

Remove many unused decls for gamma from shaders.

Make post-deferred gamma correction use display_gamma.

Make setting display_gamma use the correct RenderDeferredDisplayGamma setting.

Make fullbright rigged objects with alpha mode none use fullbright shader instead of incorrectly using the material shader.

Increase effect of variance in cloud shaders.

Modified windlight shader to remove adjustment to glow.z