mapsforge: also need to clip lines to avoid overflow
mapsforge: refactor MapDatabase

Conflicts:
	vtm/src/org/oscim/tiling/source/mapfile/MapDatabase.java
PolygonBucket: dont test bbox visibility with overzoom > 2

- workaround: seems there can be some overflow in GLMatrix.prj2d()
fix: Viewport.getDepth() for y == 0