Fix Tile X wrapping issue

[DenisCarriere]

Wraps Tile X

Wraps Tile X from pointToTile & pointToTileFraction based on the maximum Tile X allowed Math.pow(2, zoom).

Fixes issue in tile-cover mapbox/tile-cover#75

Also fixes #12

Ref: #27
CC: @tcql @mourner @lily-chai

[mourner]

This looks good, but I would rather inline the wrapping in pointToTileFraction because then you 1) reuse Math.pow calculation, 2) don't need one more temporary array

[DenisCarriere]

@mourner 👍 I didn't even realize that z2 = Math.pow(2, z) was already being calculated.

I'll update this

[DenisCarriere]

@mourner Updated the PR, was able to reduce everything down to only:

// Wrap Tile X
x = x % z2
if (x < 0) x = x + z2