handle 3d context losses

[tmcw]

there is a possibility for a rendering context to get "lost" due to certain events like low memory, browser tab changing or any other reasons. in those cases, the browser sends a "context lost" event that we should handle. in this case, we have to reacquire the context and begin setting up the map again. shouldn't be too hard. there's also an API that simulates context losses for debugging purposes.