Revert "views/month-view: Remove offset"

For a while, we have been setting child visibility to false, which
hid the horizontal line separators. However, I started scrolling in
the month view today and came to realize that they were disappearing
sporadically when scrolling upwards.

This happens because we use the `box-shadow` CSS property to apply the
box shadow a pixel above the month view row, which keeps it visible
in the view even when the month view row itself is no longer in
the page. Adding a pixel fixes that.

This reverts commit f4702dde1f.

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

@@ -1657,7 +1657,7 @@ gcal_month_view_size_allocate (GtkWidget *widget,
   gtk_widget_measure (self->header, GTK_ORIENTATION_VERTICAL, width, &header_height, NULL, NULL, NULL);
   gtk_widget_allocate (self->header, width, header_height, baseline, NULL);
 
-  grid_height = height - header_height;
+  grid_height = height - header_height + 1;
   row_height = grid_height / (gdouble) N_ROWS_PER_PAGE;
   row_scroll_offset = self->row_offset * row_height;
   y_offset = header_height - row_scroll_offset - (grid_height * ((N_PAGES - 1) / 2.0));