improved line cover algo

[morganherlocker]

Trading a bit of performance, we can get better accuracy and cover more edge cases.

given segment: x0, y0, x1, y1

1. get the tile at x0, y0
2. get the tile at x1, y1

//get intersections with verticals

3. while i <= tile2[0]
4. get the intersection of the right vertical edge of a tile at x+i
5. foreach intersection add tile [x+i-1, Math.floor(y), max_zoom] and tile [x+i, Math.floor(y), max_zoom]

//get intersections with horizontals

4. while i <= tile2[1]
5. get the intersection of the right bottom edge of a tile at y+i
6. foreach intersection add tile [Math.floor(x), y+i-1, max_zoom] and tile [Math.floor(x), y+i, max_zoom]

[morganherlocker]

done https://github.com/mapbox/tile-cover/commit/10f18436518900879b7684221e6f7b476b8a1445