SL-17705 Backwards compatibility pass.  Support OpenGL pre-4.0 by disabling reflection probes and anti-aliasing.  Get render parity with current release viewer when reflection probes are disabled.

SL-18190 Don't allocate mips for irradiance maps because they're never generated.  Disable OpenGL core profile on Intel by default.

SL-18095 WIP -- Add Mikktspace tangent generation for PBR materials and switch to per-pixel binormal generation.  Still bugged with some test content.

SL-17005: (WIP) (Debug) Add logging and debug settings to assist in understanding Windows 8 failure modes. Most of this should be removed later

SL-17574 Add probe detail combo box to advanced graphics preferences.  Fix spot light shadows not working in probes.

SL-17573 Add "dynamic" checkbox, also followup on SL-17551 and do "Select Invisible Objects" checkbox instead of "Select Reflection Probes"

SL-17551 Add "Select Reflection Probes" menu option and make invisible objects less annoying when alt-zooming in edit mode.

Remove all references to LLQueuedThread (but emulate a couple bits of its API
such as handle_t and getPending()).

Migrate ImageRequest into llimageworker.cpp. It has never been part of
LLImageDecodeThread's public API. Remove ImageRequest tests.

Remove all references to LLImageDecodeThread::pause(). The idea of pausing
another thread is bizarre to me, and LLThreadPool has no such operation. Nor
does it have an abortRequest().

Now that LL::ThreadPool's constructor has subsumed the work of discovering a
runtime override width, LLAppViewer::initGeneralThread() can simply pass the
compile-time default width instead of redundantly checking ThreadPoolSizes.

Also the default ThreadPool capacity has been bumped up to what "General" and
"LLImageGL" were requesting, so they need not pass that explicitly or explain
in comments why they're doing it.

But until we start throwing work at the "General" ThreadPool, configure it
down to 1 thread in settings.xml.

SL-17472 Add checkbox to preferences for enabling "PBR" that uses "class3" deferred shaders.  Move reflection probe shaders to class3 and restore class2 shaders to non-reflection probe versions.

RenderDebugGL was avaliable from develop menu, but since it was persistent, caused slowdowns and crashes it could result in users crashing or lagging without knowing the cause. To resolve the issue it was made to last for a single session.

Due to changes in SL-16715

+ post merge conflict fix

SL-16535 FIXED The Search floater opens without appropriate search results after performing a search from the NavBar

SL-16202 Put Multi-threaded GL behind a feature flag and update featuretable (decruftify settings, compatibility pass).