Align WebMercatorQuad TMS with mercantile and GDAL

[vincentsarago]

as mentioned in #77 and #63 the WebMercatorQuad TMS which is aligned with the OGC doc is not as precise as the one used in GDAL or mercantile (it all comes down to the definition of the earth half size

# GDAL
https://github.com/OSGeo/gdal/blob/35c07b18316b4b6d238f6d60b82c31e25662ad27/gcore/tilematrixset.cpp#L79
>> 20037508.342789244

# Morecantile 
https://github.com/developmentseed/morecantile/blob/master/morecantile/data/WebMercatorQuad.json#L9
>> 20037508.3427892

Pro

• align with mercantile
• alight with GoogleMapsCompatible TMS

Cons

• COG created with rio-cogeo's Web Optimized will have different origins (but with the change, they should be aligned with GDAL ones)
• TiTiler tiles might be slightly different

cc @geospatial-jeff