增加随机读取的地形数据

[Clarmy]

根据传入的经纬度范围返回合适分辨率的地形数据并绘图

参考资料：

1. https://github.com/tangrams/heightmapper
2. https://www.nextzen.org/

[Clarmy]

https://github.com/tangrams/heightmapper/blob/8af1bd68dc41481e1c4cdf2a057d3eb68146f0be/main.js#L254

[Clarmy]

https://www.mapzen.com/blog/elevation/

Terrarium format PNG tiles contain raw elevation data in meters, in Mercator projection (EPSG:3857). All values are positive with a 32,768 offset, split into the red, green, and blue channels, with 16 bits of integer and 8 bits of fraction. To decode:

(red * 256 + green + blue / 256) - 32768

[Clarmy]

该功能将分流到另一个项目：https://github.com/Clarmy/pyterrain