HDRI Local Preview (#953)

* #926 WIP - HDRI import prototype v0

* #926 WIP -- add OpenEXR to autobuild.xml

* #926 WIP -- Add OpenEXR cmake

* #926 WIP -- Attempt at using OpenEXR autobuild package and don't hard code .exr file to load

* #926 Unmangle autobuild.xml and get dll's in the right place (thanks, Caladbolg!)

* implement mac shared libs plumbing for OpenEXR for secondlife/viewer#926

* Fix Xcode/clang compile error regarding new[]/delete[] mismatch

* #926 HDRI Preview finishing touches.

- Full ACES when HDRI is enabled
- Fix for probes getting stuck paused
- Add exposure and rotation controls

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Co-authored-by: Brad Linden <brad@lindenlab.com>