fix possible concurrency bug

[asvarga]

Instead of Update both reading and writing from TrailMap, it now reads from DiffusedTrailMap and writes to TrailMap. This seems to avoid concurrency issues because it fixes #2 (at least for me).

[asvarga]

Two things worth noting:

• Executions of TrailMap[int2(newPos)] = min(1, oldTrail + agent.speciesMask * trailWeight * deltaTime); in Update commute, so we don't have to worry about race conditions there.
• Diffuse does exactly one write to DiffusedTrailMap per pixel, so we don't have to do any other clears/copies.