Compute webmercator resolution with full accuracy

[jgoizueta]

Fixes #241

[jgoizueta]

Please review @rafatower

[rafatower]

okay, but two questions:

• how this affects actual calculations? I mean, the pixel resolution is not the same in both axis, so an error is granted depending on the zoom level, right? (from my ignorance).
• have you tested what happens with the IMMUTABLE modifier when you update the extension? Will it actually take the newest, more accurate version of the function's results?

[jgoizueta]

Basically we're adding a small (irrelevant for current applications) but unnecessary error to the inevitable error due to floating point's limited precision. Not a big deal, but this makes the exact values computed inconsistent with those used in the tiler: https://github.com/CartoDB/tilelive-mapnik/blob/d04792444eb356d1e240ecd792f5d0ab933b213e/lib/render.js#L7

Regarding the immutability, I've checked that when the function is replaced the new values are used (even for z values previously computed)

[rafatower]

👍