Max zoom 22

[fdansv]

Ref. CartoDB/torque#55

Carto has a max zoom value of 22. Changing that value produces unexpected results, so obviously there is more to it.

@javisantana
@tiangolo

[pnorman]

at z22 there are 2^30+buffer pixels, at z23 there are 2^31+buffer pixels, so use of 32 bit integers to represent pixel coordinate from 0,0 will fail.

All sorts of parts most tile stack have implicit assumptions like this which break down at z23.

fyi, at middle-north latitudes, z22 is 2.5cm/px