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

diff --git a/gtk/gtkimcontextsimple.c b/gtk/gtkimcontextsimple.c
index 38806d0..2cb2289 100644
--- a/gtk/gtkimcontextsimple.c
+++ b/gtk/gtkimcontextsimple.c
@@ -1128,7 +1128,7 @@
     }
 
   if (cursor_pos)
-    *cursor_pos = s->len;
+    *cursor_pos = g_utf8_strlen (s->str, s->len);
 
   if (attrs)
     {