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.
Use the same API/pattern for parsing raster tiles as we use for vector
tiles. Forward the error as an exception that will ultimately reach the
StillImageCallback.
Also set the tile to State::obsolete on error like we do for vector
tiles.
