Fix bug in multires tile loading, which primarily affects Safari (#1012).
Fix bug in multires tile loading, which primarily affects Safari (#1012).

Bug was introduced in 5eab5366f6987a4abdf648cbf5f7a243f14faf28. It caused tile
loader objects of non-web-worker-based tile loading method to sometimes be
lost during scene changes.

mpetroff/pannellum,mpetroff/pannellum,mpetroff/pannellum,mpetroff/pannellum