Improve visible placemark caching. Lazy label pixmap creation.

Deleted placemarks (e.g. tile removed in vector osm) were kept in
the cache forever with their visible placemark counterpart only being
deleted on a style change (~map theme change). Placemarks that moved
out of the viewport but not being deleted directly however had their
visible placemark counterpart deleted directly, leading to a costly
recreation when the placemark moved back into the viewport soon after.

Now visible placemarks are cached only in an area around the viewport,
and eventually cleaned up when their placemark counterpart is deleted.

Lazy label pixmap creation is useful since some placemarks will never
have their label shown, its creation however is rather costly due to
the painter path involved (with glow style).