-Many GLES2 optimizations

-Android export fixes (use ETC if GLES2 backend in use)
-revert to thekla atlas because xatlas is not working well

modules/xatlas_unwrap/config.py

@@ -1,5 +1,6 @@
 def can_build(env, platform):
-    return (env['tools'] and platform not in ["android", "ios"])
+    return False #xatlas is buggy
+    #return (env['tools'] and platform not in ["android", "ios"])
 
 def configure(env):
     pass