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).