bug in salva3d::sampling::shape_surface_ray_sample() with a heightfield

[rezural]

sampling the surface of a heightfield, causes lines to appear in the sampled partices, where it should be a continuous 'plane' of particles.

This is an example, where there should be 100 particles, but only 81. It usually happens along an axis, but sometimes at 45 deg angles (I havent been able to repro that).

Repro here:

https://github.com/rezural/heightfield-raycast-bug