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/platform_impl/linux/wayland/seat/text_input/mod.rs

@@ -119,11 +119,15 @@ impl Dispatch<ZwpTextInputV3, TextInputData, WinitState> for TextInputState {
                     None => return,
                 };
 
-                // Clear preedit at the start of `Done`.
-                state.events_sink.push_window_event(
-                    WindowEvent::Ime(Ime::Preedit(String::new(), None)),
-                    window_id,
-                );
+                // Clear preedit, unless all we'll be doing next is sending a new preedit.
+                if text_input_data.pending_commit.is_some()
+                    || text_input_data.pending_preedit.is_none()
+                {
+                    state.events_sink.push_window_event(
+                        WindowEvent::Ime(Ime::Preedit(String::new(), None)),
+                        window_id,
+                    );
+                }
 
                 // Send `Commit`.
                 if let Some(text) = text_input_data.pending_commit.take() {