Rigid body physics: Move effector force update into substep loop.

The substep loop for rigid bodies causes unequal effects of force fields depedending on the substep setting, larger
substep counts cause a diminishing effect of force fields.
This is because the force to apply on a body is reset in Bullet after each step and needs to be recomputed. Without this
the body will just coast with constant velocity after the first substep. Since the per-step impulse with larger substep
counts is smaller, the effect is that more substeps cause a smaller total impulse.

The fix is to move external force calculation into the substep loop and update forces for each substep.

Note that this may be considered a breaking change, because the breaking commit rB1aa54d4921c2 has been in master for
a long time and after this fix force fields will generally have a much larger effect on rigid bodies (10x for the
default setting of 10 substeps).

Differential Revision: https://developer.blender.org/D15173

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