apprt/gtk: any preedit change should note a composing state

Fixes #6772

When typing Korean with the fcitx5-hangful input method, moving between
graphemes does not trigger a preedit end/start cycle and instead just
clears the preexisting preedit and reuses the started state.

Every other input method we've tested up until now doesn't do this. We
need to mark composing set to "false" in "commit" because some input
methods on the contrary fail to ever call END.

What is the point of start/end events if they are just ignored depending
on the whim of the input method? Nothing. That's what. Its all a mess
that GTK should be protecting us from but it doesn't and now its the app
developer's problem. I'm frustrated because I feel like the point of an
app framework is to mask this kind of complexity from the app developer
and I'm playing whack-a-mole with input methods.

Well, here's another whack. Let's see if it works.

src/apprt/gtk/Surface.zig

@@ -1935,6 +1935,14 @@ fn gtkInputPreeditChanged(
 ) callconv(.C) void {
     const self = userdataSelf(ud.?);
 
+    // Any preedit change should mark that we're composing. Its possible this
+    // is false using fcitx5-hangul and typing "dkssud<space>" ("안녕"). The
+    // second "s" results in a "commit" for "안" which sets composing to false,
+    // but then immediately sends a preedit change for the next symbol. With
+    // composing set to false we won't commit this text. Therefore, we must
+    // ensure it is set here.
+    self.im_composing = true;
+
     // Get our pre-edit string that we'll use to show the user.
     var buf: [*c]u8 = undefined;
     _ = c.gtk_im_context_get_preedit_string(ctx, &buf, null, null);