This eliminates the reliance on ThreadContext to provide GLObjectStore, and statically enforces that GL cleanup functions happen only when GLObjectStore::performCleanup is called.
With the elimination of the Map thread, this becomes important because there may be multiple GLObjectStore's per-thread, and Map will need to ensure that the correct context is active when calling GLObjectStore::performCleanup.
mbgl::Thread will keep a ThreadContext for each running instance
in a thread_local so we don't need to lookup a man in the Environment
every time we need some info about the current thread.
This patch is moving the ::currentlyOn check used on Debug build from
the Environment class to the ThreadContext.
