(cherry picked from commit 19c19e20)

Spatially adjacent regions (e.g. neighboring states) share borders
such that in rectangular tiles data from both appears. These need to
be handled individually in order to have data from both (or more)
input files appear in the final tile.

(cherry picked from commit f6618ae6)

Same approach as in GeoGraphicsScene. Placemarks are assigned a tile
level that fully contains them once. When clipping to some boundary
box, only those placemarks on the path towards the top of the pyramid
can intersect them. Note that this is only a heuristic; checking
the actual bounding box intersection again afterwards still rules
out a lot of placemarks that don't actually intersect.

Rough speed comparison for different approaches:
A) Slow:      clip all placemarks directly
B) Medium:    clip all placemarks based on tile pyramid
C) Fast:      clip all placemarks that intersect with the bounding box
D) Very fast: clip all placemarks based on tile pyramid that also
              intersect with the bounding box

C) is the old implementation, B) something I tested locally and C) the
new implementation.

Creating vector tiles locally is now as easy as running
marble-vectorosm-tilecreator http://download.geofabrik.de/europe/germany/baden-wuerttemberg/bremen-latest.osm.pbf

Local files can be passed as well. See --help for additional options.

Input map files are usually the largest files and traversing them
instead of output tiles ensures they have to be loaded into memory
just once

Typical usage:
marble-vectorosm-tilecreator -o <dir> -z 11,13,15 -m landmass/7 map.o5m

Where landmass/7 is a directory created before using e.g.
marble-vectorosm-tilecreator -o landmass -z 7 -k landmass.shp
(note -o landmass and -z 7 resulting in landmass/7/... tiles)

To avoid reading in a large landmass file for the whole world, they
should be preprocessed into tiles instead. Typical usage:
marble-vectorosm-tilecreator -o <dir> -z 7 -k landmass.shp

Since we display tile level n up to zoom level n+1, reduce nodes
for level n+1, not n.