Blender Git Loki

Fix T90346: particle force field self effect amount off by one

When calculating the particle step in `get_effector_tot`, we have to
round up (otherwise we might get an extra round in the for-loop in
`BKE_effectors_apply` for certain cases).

Example from the report:
- 10.000 particles, Effector Amount 3
- was rounding the step down to 3333
- going into the for-loop for 0, 3333, 6666 and 9999 (4 times)
- now rounding the step up to 3334
- going into the for-loop for 0, 3334 and 6668 (3 times as desired)

Maniphest Tasks: T90346

Differential Revision: https://developer.blender.org/D12113