New landscape type: custom horizon line
A new landscape type that renders a simple, monochrome landscape, defined as series of points with different elevation above the horizon, similar to the "horizon" feature in SkyChart/Cartes du Ciel:
The horizon is defined as a series of (azimuth, altitude) pairs, where the altitude is given in degrees above the horizon.
Stellarium may render the landscape in a single colour, or in two - one for the base (below the horizon), and one for the "mountains". (The second variation is used in Cartes du Ciel.)