With TileId instances as key for loaded and queued documents, their
map theme id must match always.

- 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).

- Improves rendering since street names are better layed out and no
  false split indicators are drawn
- Improves render performance (less items to paint)
- Drawback of course is increased code complexity and a small
  overhead for merging strings. Seems reasonable though.

Note that due to clipping, this results in duplicated ids across
several tiles were the geometry differs until all features with the
same id have their geometries merged.

This allows to control the location of the Qt installation with
a single -DQt5_DIR=... instead of having to pass this for every
Qt module.

The visual category now decides which one to render in case several
placemarks collide on screen.