Instead of pointer to pointer. The two level of pointers were used to
inform the caller the window has been destroyed (pointer set to NULL).
Now, destruction of windows are now signaled by the return value, which
is marked `must_use`.
Problem with taking pointer to pointer is that they tends to pollute the
function signatures a little bit. And we are not using it well anyways.
A lot of the callers passes pointers to local variables to those
functions, so the information about window destruction doesn't actually
bubble up. So, we switch to use just one level of pointers instead.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
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