Don't use emscripten.async_call while already in async mode

This particular use case appears to be safe, but I could make Ren'Py crash with more complex async_call-s.
This may also explain some irregular crashes a while ago when letting RenPyWeb run unattended.
This is somewhat similar to running Python in two threads. In doubt, let's avoid.
Moreover, however autosave is run, the browser will freeze a bit, no significant change.

renpy/loadsave.py

@@ -570,8 +570,7 @@ def force_autosave(take_screenshot=False, block=False):
         autosave_thread.daemon = True
         autosave_thread.start()
     else:
-        import emscripten
-        emscripten.async_call(autosave_thread_function, take_screenshot, -1)
+        autosave_thread_function(take_screenshot)
 
 
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