Reduce tile-join overzooming memory usage (#162)

* 16 bits is enough for tile numbers

* Revert "16 bits is enough for tile numbers"

This reverts commit 71a0c4e1cf.

* Check what child tiles each overzoomed tile will have

and don't queue further overzooming of empty tiles

* Clean up naming; use std::move to avoid copying large arrays

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 2.37.0
+
+* Speed up tile-join overzooming and make it use less memory, by not including empty child tiles in the enumeration
+
 # 2.36.0
 
 * Make tile-join distrust the source tilesets' metadata maxzoom and minzoom