Allow ChunkGenerateEvent-like rendering behavior for pre-generated maps

[jpenilla]

Basically, the goal is to allow behavior how you would get without a pre-generated map and ChunkGenerateEvent, where chunks render once a player goes near them; but with a pre-generated map and without the performance penalty of the current PlayerChunkLoadEvent option (which would be the current workaround to accomplish this, and will cause lots of unnecessary renders).

To implement this we want to add a new map update listener "player map discovery" or something, which will queue chunks for render on player chunk load, give the chunk a persistent marker that it's "discovered", and then ignore any "discovered" chunks in future player load events. We could also go further and ignore non-discovered chunks in other update listeners, but that can be considered after the base feature is implemented.