Reinstate functionality of Sphere mirror probes

Environment

Second Life Release 7.1.8.9375512768 (64bit)
Release Notes

CPU: Intel(R) Core(TM) i7-5930K CPU @ 3.50GHz (3491.92 MHz)
Memory: 32610 MB
OS Version: Microsoft Windows 10 64-bit (Build 19045.4412)
Graphics Card Vendor: NVIDIA Corporation
Graphics Card: NVIDIA GeForce GTX 1080/PCIe/SSE2

Windows Graphics Driver Version: 31.0.15.3623
OpenGL Version: 4.6.0 NVIDIA 536.23

Window size: 1506x1184
Font Size Adjustment: 96pt
UI Scaling: 1
Draw distance: 128m
Bandwidth: 3000kbit/s
LOD factor: 1.75
Render quality: 5
Texture memory: 7304MB
Disk cache: Max size 1638.4 MB (58.6% used)

J2C Decoder Version: KDU v7.10.4
Audio Driver Version: FMOD Studio 2.02.20
Dullahan: 1.14.0.202310131404
CEF: 118.4.1+g3dd6078+chromium-118.0.5993.54
Chromium: 118.0.5993.54
LibVLC Version: 3.0.16
Voice Server Version: Not Connected
June 05 2024 11:30:58

Description

Reinstate functionality of Sphere mirror probes. Sphere mirror probes have been disabled to eliminate flickering described in https://github.com/secondlife/viewer/issues/1628

Reproduction steps

See https://github.com/secondlife/viewer/issues/1628

Start by reading issue 1628, which describes the flickering that led to sphere mirror probes being disabled. Trace the viewer's sphere mirror probe rendering entry points from there and determine how functionality can be restored without reintroducing that flicker. Done means sphere mirror probes work again and the reported flickering is addressed.