st/entry: Invalidate shadow on cursor position/size changes

When text shadows are used, the cursor indicating the current position
also casts a shadow.

This means we have to regenerate the shadow texture after the cursor
position changed, so invalidate the shadow in that case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1744>

src/st/st-entry.c

@@ -539,7 +539,11 @@ static void
 clutter_text_cursor_changed (ClutterText *text,
                              StEntry     *entry)
 {
+  StEntryPrivate *priv = ST_ENTRY_PRIV (entry);
+
   st_entry_update_hint_visibility (entry);
+
+  g_clear_pointer (&priv->text_shadow_material, cogl_object_unref);
 }
 
 static void