Avoid precision issues with integer ray coordinates

Improve precision of labeling algorithm by switching to integer coordinates in GridLattice

Using double coordinates in the lattice lost information about which exact grid edge has been detected as intersected. This causes inconsistency issues when the intersection point lay on the grid vertex, because the intersected edge could have been confused with its neighboring one due to numerical errors.

Keeping integer coordinates coming from the grid itself avoids this issue as now comparisons are exact.