wayland: clear IME preedit only when necessary

When all we'll be doing is setting a new preedit, the preedit doesn't
have to be explicitly cleared first. This change is perhaps debatable.

The direct reason for this is to make it easier to work around
quirks/bugs: in Masonry we've found IBus appears to resend
the IME preedit in response to `Window::set_ime_cursor_area`
(`zwp_text_input_v3::set_cursor_rectangle`). Because currently the
preedit is first cleared, a new IME cursor area is sent, which again
causes IBus to resend the preedit. This can loop for a while.

The Wayland protocol is mechanically quite prescriptive,
it says for zwp_text_input_v3:event:done.

> 1. Replace existing preedit string with the cursor.
> 2. Delete requested surrounding text.
> 3. Insert commit string with the cursor at its end.
> 4. Calculate surrounding text to send.
> 5. Insert new preedit text in cursor position.
> 6. Place cursor inside preedit text.

Winit currently doesn't do surrounding text, so 2. and 4. can be
ignored. In Winit's IME model, without a commit, sending just the
`Ime::Preedit` event without explicitly clearing is arguably still
equivalent to doing 1., 5., and 6.

src/changelog/unreleased.md

@@ -39,3 +39,7 @@ The migration guide could reference other migration examples in the current
 changelog entry.
 
 ## Unreleased
+
+### Changed
+
+- On Wayland, no longer send an explicit clearing `Ime::Preedit` just prior to a new `Ime::Preedit`.