IME editing would cast GHOST_TEventImeData to wmIMEData then read/write
an additional member that doesn't exist in GHOST_TEventImeData.
In practice it's likely struct padding prevented this from showing up
as a bug. Nevertheless it's bad practice to rely on this.
- Make GHOST_TEventImeData read-only, move the is_ime_composing boolean
into the window.
- Add static assert to ensure both structs are the same size.
- Correct code comments.
0742ef808f