patch 8.0.1026: GTK on-the-spot input has problems

Problem:    GTK on-the-spot input has problems. (Gerd Wachsmuth)
Solution:   Support over-the-spot. (Yukihiro Nakadaira, Ketn Takata, closes
            #1215)
diff --git a/src/screen.c b/src/screen.c
index b576214..c3bcd3b 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -5197,7 +5197,8 @@
 	/* XIM don't send preedit_start and preedit_end, but they send
 	 * preedit_changed and commit.  Thus Vim can't set "im_is_active", use
 	 * im_is_preediting() here. */
-	if (xic != NULL
+	if (p_imst == IM_ON_THE_SPOT
+		&& xic != NULL
 		&& lnum == wp->w_cursor.lnum
 		&& (State & INSERT)
 		&& !p_imdisable