Fix #111: Voronoi tessellation repeats along x-axis on Windows

[JonathanMaes]

As reported in #111, the Voronoi tessellation exhibited periodicity along the x-axis on Windows.
The culprit is engine_.seed(seed ^ seed_); on line 127 in src/core/voronoi.cpp.

Cause

The Voronoi tessellation uses "tiles" for efficiency: the RNG generates some Voronoi centres in each of these. The seed for this RNG is based on the (x,y,z)-index of the current tile: it was a 64-bit number constructed by taking the last 24 bits of the x-index, the last 20 bits of the y-index, and the last 20 bits of the z-index. The issue was that the argument of engine_.seed(...) has a different data type in Windows than on Linux:

• MSVC's default RNG is std::mt19937, seeded using an unsigned int,
• GCC's default RNG is std::minstd_rand0, seeded using a uint_fast32_t.

Somehow, passing a 64-bit seed on MSVC cuts off the first 32 bits, while on GCC the 64-bit number stays intact. Hence, on Windows, the x-index of the tile was completely disregarded, explaining the periodicity along x.

Fix

The seed was changed to a 32-bit integer consisting of 12 x-, 10 y-, and 10 z-bits. Hence, the periodicity now occurs over a length scale of ⪆2048 Voronoi cells, rather than 2 as was the case for the x-axis. I believe this should suffice for all but the most extreme simulations.