deploy-stub: chdir to Resources dir in GUI macOS app bundle

This prevents a common error of trying to load non-Panda3D assets without specifying $MAIN_DIR.  We aren't currently making any guarantees about the cwd (which may in fact be / when launching from Finder), so I think this is safe.

pandatool/src/deploy-stub/deploy-stub.c

@@ -522,6 +522,10 @@ int Py_FrozenMain(int argc, char **argv)
     // for ConfigPageManager to read out and assign to MAIN_DIR.
     sprintf(buffer, "%s/../Resources", dir);
     set_main_dir(buffer);
+
+    // Finally, chdir to it, so that regular Python files are read from the
+    // right location.
+    chdir(buffer);
 #endif
 
     n = PyImport_ImportFrozenModule("__main__");