views/month: Only recalculate layout blocks when needed

The tick callback can be fired *after* the GtkWidget.map() vfunc is
called, in which case the recalculate_layout_blocks() will run with
an already recalculated layout, which is invalid.

Only call recalculate_layout_blocks() when the blocks are invalidated.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/768>

src/gui/views/gcal-month-view-row.c

@@ -930,7 +930,8 @@ widget_tick_cb (GtkWidget     *widget,
 
   g_assert (GCAL_IS_MONTH_VIEW_ROW (self));
 
-  recalculate_layout_blocks (self);
+  if (!self->layout_blocks_valid)
+    recalculate_layout_blocks (self);
 
   GCAL_RETURN (G_SOURCE_REMOVE);
 }