Terrain discovery mode (background and/or on command)

[Majiir]

A command should explore for existing terrain by attempting to load chunks without generating them. A threshold controls how far beyond existing terrain the command may explore before terminating. The exploration pattern should minimize the number of chunks loaded while ensuring that adjacent chunks are preprocessed before rendering a given chunk.

An example pattern is as follows:

• The plugin explores in spirals, starting at the origin
• Each spiral has a width of 16 chunks
• Each spiral additionally loads one chunk further inside the innermost chunk to expose new height map information
• The front row of 16 chunks loads before the row immediately before it is unloaded.
• The pattern terminates when the spiral radius reaches two times the radius of the furthest discovered terrain

[Majiir]

Experimental system with a simple spiral pattern implemented in commit 805aad2f8be2b2daa01b6188e06316dd3d6a889c.