Batch Rendering for placemark symbol pixmaps:

- Actually share identical pixmaps in memory
- Cache them inside a dedicated QPixmapCache
- Render identical symbols in a batch using
  QPainter::drawPixmapFragments()

Batch rendering can be disabled via the
BATCH_RENDERING define.

The "O"-keystroke-debug mode is now extended
by displaying a background color for each
placemark. Placemarks rendered in parallel
share the same background color.

This should benefit rendering of huge amounts of
pixmaps of the same kind (e.g. trees).