More optimizations, nicer alarm background

- Alarms now get two red (oxygen brick red 5) gradients from top and bottom.
This makes colors of departures better recognizable with enbled alarms.
- Mix the background color and fill the buffer pixmap with it,
instead of first filling with Qt::transparent and then drawing rects with
transparent colors one over the other.
- Don't call QTextDocument::setDefaultFont in each call to
DepartureGraphicsItem::paintItem, only if the font changed (when another stop
gets highlighted).