input: Cancel pointer grabs when compositor loses kb focus

This makes sure we pop down popups when the X backend loses keyboard
focus or when the kms backend vt-switches away.
diff --git a/src/input.c b/src/input.c
index 06a3c2e..ae0e832 100644
--- a/src/input.c
+++ b/src/input.c
@@ -1297,6 +1297,8 @@
 
 	weston_keyboard_set_focus(keyboard, NULL);
 	weston_keyboard_cancel_grab(keyboard);
+	if (seat->pointer)
+		weston_pointer_cancel_grab(seat->pointer);
 }
 
 WL_EXPORT void