SH-2244 Fix for crash when transparent water disabled (silently ignore setting when basic shaders enabled)

SH-2244 Vertex buffer cleanup, fix for bad vertices in rigged attachments, added "RenderUseVAO" debug setting to control whether or not to use VAO's in non-core GL profiles.

SH-2244 Fix "RenderGLCoreProfile" actually make a core profile context and modify viewer to run under said context without generating errors.

SH-2244 Add "RenderGLCoreProfile" debug setting that allows the viewer to start with a non-compatibility-profile OpenGL context.

SH-2243 work in progress -- don't use gl_FragColor or gl_FragData for outputs -- no more deprecation warnings on nvidia

STORM-1562 Potential fix for crash in LLCurl::run -- make sure all ref counted members of llsecapi are thread safe

STORM-1562 Potential fix for crash in LLCurl::run -- don't run curl_multi_perform from multiple threads simultaneously.

Now that we have unit tests that require assertion failure if you try to
delete an LLInstanceTracker subclass instance with an iterator loose, having
llassert() "sometimes" compile away (whimsically, depending on platform as
well as build type!) makes those tests fail. Use llassert_always() instead.