Add Chinese Gaode Map base layer

[mutongx]

Gaode Maps is a map service providing detailed and accurate map of China. Google Maps' data is much outdated and OSM is almost blank in some area of China.

This map layer includes loading map tiles from Gaode's server and fixing offset of the GCJ02 coordinate system.

[pleasantone]

This version of the 'box' has proven problematic before, as it covers large areas that are not China.
Can you confirm that this map utilizes GCJ even for areas outside of china?
e.g. Bhutan, for example, is inside your grid, but uses WGS84, not GCJ... please just verify that it works right.

[mutongx]

Actually Gaode Maps have no map data outside China. You can see that there's only a boundary line around Bhutan and inside the country it's blank. I believe Chinese government would apply the same GCJ-02 offset to the whole Gaode Map but I think there is no way to confirm it because their algorithm is not public (the algorithm on the Internet is just a approximation).
I can confirm that map data of Hong Kong, which uses WGS-84 in Google Maps, uses GCJ-02 in Gaode Maps, and portal's locations are accurate.

[mutongx]

Someone reported that Gaode in Taiwan uses WGS-84 and I confirmed it's true. Fixed now.
I think this base layer should be China-only. If you zoom in you will see nothing outside China, even satellite images, so there's no need to worry about other areas as far as I'm concerned.

[Artoria2e5]

Hi, in #1188 I have a polygon instead of a box to make things better. That version of "China, but just the distorted part" does not include Taiwan, Hong Kong, and Macau; you may want to cut off the "hkmo" part in my comments to include HK and Macau. Anyway, you are free to use that polygon instead of a box.