SL-19564 Rebalance exposure and sky.  Hack legacy diffuse map saturation and brightness to allow ACES Hill all the time.

* SL-19538 Followup -- fix for dynamic exposure having large gaps in its luminance sampling.

* SL-19538 Followup -- review feedback changes.

SL-18869 Optimizations -- Decruftify LLRenderTarget, use a shader to copy color/depth instead of glCopyTexSubImage or glBlitFrameBuffer

SH-2908 Rework indexed texture rendering to use a uvec4 instead of a float for texture indices in the data stream.  Also rework gl_FragColor overrides to not collide with some odd driver implementations.

SH-2243 work in progress -- don't use gl_FragColor or gl_FragData for outputs -- no more deprecation warnings on nvidia

SH-1682 Dynamically adjust the number of texture channels to use for indexed texture rendering based on available hardware.

See https://wiki.lindenlab.com/wiki/License_Substitution for details.

windlight14 which have passed QA (up through r79932).

svn merge -r 80831:80833 svn+ssh://svn.lindenlab.com/svn/linden/branches/merge_windlight14_r80620