Today, the Rich Edit control handles the WM_IME_STARTCOMPOSITION message
by the computing the composition start position (imeStartIndex) _before_
calling ME_DeleteSelection(), which shifts the character positions after
the range of deletion.  If the selection were not empty, imeStartIndex
immediately becomes stale, since it does not take into account the
number of deleted characters before it.

Fix this by computing imeStartIndex after the ME_DeleteSelection() call.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=54318
(cherry picked from commit 8c43577f)