world: also allow chunks of status "spawn"

Apparently this is a status sometimes seen with worlds converted from
1.12 to 1.14. From the sample I have, it looks like they're properly
populated with light and terrain data, so hopefully this doesn't break
anything.

Possibly fixes #1630.

overviewer_core/world.py

@@ -1339,7 +1339,7 @@ class RegionSet(object):
         # to SkyLight not being calculated, which results in mostly-black chunks,
         # so we'll just pretend they aren't there.
         if chunk_data.get("Status", "") not in ("full", "postprocessed", "fullchunk",
-                                                "mobs_spawned", ""):
+                                                "mobs_spawned", "spawn", ""):
             raise ChunkDoesntExist("Chunk %s,%s doesn't exist" % (x,z))
 
         # Turn the Biomes array into a 16x16 numpy array