Optimization: Re-use tile URL regex in BlueprintInputGeoJSON

Creating a new `RegExp` every time `requestTiles` is called is slow. It was one of the pain points that showed up in the browser script profiler.

This optimization is one of many necessary to get the frame rate consistent enough for virtual reality applications (#57). This may not make much difference in the basic example, but it becomes evident when new tiles are requested more frequently.