Added xatlas as alternative to thekla, forced it on.

Did some hacks to it to avoid it from failing on bad geometry.

modules/xatlas_unwrap/config.py

@@ -0,0 +1,5 @@
+def can_build(env, platform):
+    return (env['tools'] and platform not in ["android", "ios"])
+
+def configure(env):
+    pass