reenable QPainter+ClipPainter methods by using C++'s "using" keyword rather than by reimplementing the methods
no need for taking radius of atmosphere into account when drawing the globe
align ternary operators like others

* makes code more readable
pass a GeoDataLatLonBox instead of ViewportParams
rename VectorTileMapper -> VectorTileModel

* nothing is mapped, so model is probably the better name
make panning of tiled vector maps much smoother and add support for multiple layers

* vector tiles are now loaded in the background
* support for multiple layers came almost for free by shifting code around
* unfortunately, panning doesn't become ultimately smooth since GeoDataTreeModel is reloaded in the GUI thread when a vector tile is added/removed
remove vector tile capabilities from texture mapping classes

From an architectural point of view, it's not a good idea to "hijack" the texture classes with vector capabilities.
This can be seen by various type-unsafe if-statements scattered througout the code.
Fortunately, this is history now.
introduce MarbleView
introduce MarbleGLView
introcude GlLayerInterface
clear depth buffer between layer rendering
TextureLayer
PlacemarkLayer
introduce triangulation algorithm for polygons

* taken from http://www.crystalspace3d.org/svnroot/crystal/CS/trunk/{include/csgeom/triangulate.h, libs/csgeom/triangulate.cpp}.
GeometryLayer
introduce GeoGraphicsScene::GlTile and use glDrawElements
adapt PlacemarkLayout
use shaders
GLRenderer