Limit the number of features drawn

We shouldn't support an unbounded number of features, or we have no way of limiting our memory usage.

20,000 features in one tile typically means we're drawing pretty dense features, so we shouldn't miss the extra features. But we can adjust this as needed through an environment variable to reflect:
+ the realities of usage
+ the memory limits of the hosting environment
+ memory-saving improvements we might make down the road