imcontext: Fix cursor positions

gtk_im_context_get_preedit_string is documented to
return the cursor position as a character offset,
not a byte count. So return that.

Fixed: #3885

gtk/gtkimcontextsimple.c

@@ -1131,7 +1131,7 @@ gtk_im_context_simple_get_preedit_string (GtkIMContext   *context,
     }
 
   if (cursor_pos)
-    *cursor_pos = s->len;
+    *cursor_pos = g_utf8_strlen (s->str, s->len);
 
   if (attrs)
     {