Large cleanup of iterators inside LLSelectMgr to use range-for semantics and to reduce excessive end calls

Replace std::recursive_mutex with LLMutex inside LLSingleton as the recursive mutex impl to avoid virtual dispatch overhead of std::mutex on msvc

Replace std::mutex with absl::Mutex in LLInstanceTracker, as its dramatically faster at locking and unlocking an uncontended lock.

Fix lldynamictexture using wrong fbo
Bump Bake fbo to 1024x1024
Build fixes to the new LLModelPreview