add an elevation background

[guillaume-alvarez]

may use google elevation API to load it: https://developers.google.com/maps/documentation/elevation/intro

• request the points for all england with a resolution corresponding to final map display (at the moment 1/4800) or maybe an even bigger resolution
• use an encoded polyline to minimize the number of requests (precision does not really matter here, it is just eye candy)
• generate an image in pixi (or python?) and use it as background