Support a precise way of merging/joining splats

[attilaolah]

This feature request is a bit vague, but I'd like to be able to join a number of slaps that have overlapping objects or surfaces (mostly outdoor scenes). Ideally this could be done automatically, maybe even by a separate tool, but for lack of a better alternative, it could be done manually in SuperSplate, e.g. by selecting common points that appear in both scenes.

Similarly to how stitching works in 2D perspective images, e.g. minimise the maximum distance between points, or the mean/median distance.