following promotion of DRTVWR-570

SL-19000 Fix various 3D UI components not respecting depth buffer.  Incidental decruft.  Do I get a prize for 1000th jira?

SL-19015 Balance sun/sky ambiance with punctual light ambiance.  Prevent irradiance maps from being brighter than the environment.

This commit adds the HBXX64 and HBXX128 classes for use as a drop-in
replacement for the slow LLMD5 hashing class, where speed matters and
backward compatibility (with standard hashing algorithms) and/or
cryptographic hashing qualities are not required.
It also replaces LLMD5 with HBXX* in a few existing hot (well, ok, just
"warm" for some) paths meeting the above requirements, while paving the way for
future use cases, such as in the DRTVWR-559 and sibling branches where the slow
LLMD5 is used (e.g. to hash materials and vertex buffer cache entries), and
could be use such a (way) faster algorithm with very significant benefits and
no negative impact.

Here is the comment I added in indra/llcommon/hbxx.h:

// HBXXH* classes are to be used where speed matters and cryptographic quality
// is not required (no "one-way" guarantee, though they are likely not worst in
// this respect than MD5 which got busted and is now considered too weak). The
// xxHash code they are built upon is vectorized and about 50 times faster than
// MD5. A 64 bits hash class is also provided for when 128 bits of entropy are
// not needed. The hashes collision rate is similar to MD5's.
// See https://github.com/Cyan4973/xxHash#readme for details.

SL-19015 Bump probe resolution back to 256 by default (drop to 128 if vram < 2GB), remove irradiance map feedback loop (one bounce, but but more stable and allows for much brighter first bounce), make sky contribution to irradiance not tint the world blue.  Make irradiance that appears in radiance maps match world irradiance.

* SL-19203 WIP -- Integrate SSR with reflection probes.  Decruft LLRenderTarget.

* SL-19203 WIP -- Re-integrate SSR.  Incidental decruft.

* SL-19203 WIP -- SSR frame delta correction (still broken for Z)

* SL-19203 WIP -- SSR frame delta Z fix

* SL-19203 WIP -- Make SSR toggleable again and disable SSR in cube snapshots.

* SL-19203 WIP -- Soften sphere probe transitions and fix reflections on void water (make fallback probe a simple terrain+water+sky probe).  Remove parallax correction for automatic probes to reduce artifacts.

* SL-19203 Tune probe blending.

* SL-19203 Cleanup.

This reverts commit 4d429b7e.