Update xatlas to f65a664.

xatlas is now multicore.

thirdparty/README.md

@@ -511,7 +511,7 @@ folder.
 ## xatlas
 
 - Upstream: https://github.com/jpcy/xatlas
-- Version: git (b7d7bb, 2019)
+- Version: git (f65a664, 2019)
 - License: MIT
 
 Files extracted from upstream source: