msi: Fix the dialog centering logic.

[Mike McCormack]

This should be tested with a window manager that doesn't auto-place windows.

The existing logic seems to center the upper left corner of the dialog.

Mike

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  dlls/msi/dialog.c |   41 ++++++++++++++++++++++++-----------------
  1 files changed, 24 insertions(+), 17 deletions(-)
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