The documentation doesn't state that the argument is ever modified,
and no implementation does so currently.
This is a non-breaking change to guarantee as much to callers.
Note that this is purely to make it possible to enter text that requires
composition - for example, before this commit Kanji input didn't work at all.
The big problem this still has is that we need the window position, and this is
still not implemented. Once we have this information we can do the equivalent
of XTranslateCoordinates to put the rectangle where we want it.
