- Do not try initializing shaders if requirements are not met or if window does not exist
- Warn user before closing window. Situation is unexpected, there is chance of more issues, like a freeze or a crash on close()
- mHasMultitexture is a solid requirement, if it is not present, we won't be able to run

SL-16815 and SL-16906 Avoid redundant bumpmap generation, add some assertions around ref counting and (hack) fix crash on shutdown from dangling texture reference (reduced to 1 dangling texture from several hundred, can't find the remaining reference).

SL-16815 Cleanup -- disable multithreaded bumpmap generation while tracking down loading issues, fix sync issue in single threaded mode in media textures, restore LL_IMAGEGL_THREAD_CHECK functionality

SL-16815 Remove frame stalls from occlusion queries, bumpmap updates, and querying for available video memory.

SL-16418 Media texture update stall fix.  Make media texture updates use LLImageGL thread to update, fix AMD sync issue on ImageGL thread and install debug callbacks on LLImageGL thread when debug gl enabled.

SL-16727: Revert change causing llvieweroctree to not render reflections when shadows are set to projectors cause by commit bb379cb8

This reverts commit 40fe5277, reversing
changes made to af830e5f.

SL-16653 Fix for some textures having incorrect texel colors after the first row (and add more paranoia checks on texture data).

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

remove unused variable

Reverting a merge is sticky: it tells git you never want to see that branch
again. Merging the DRTVWR-546 branch, which contained the revert, into the
glthread branch undid much of the development work on that branch. To restore
it we must revert the revert.

This reverts commit 029b41c0.

It's sometimes important to finish other initialization before launching the
threads in the ThreadPool, so make that an explicit step. In particular, we
were launching the LLImageGL texture thread before initializing the GL
context, resulting in all gray textures.

SL-16239 Fix for slowdown on AMD GPUs (disable core profile and remove volatile members from LLVertexBuffer)