Particle system: Move runtime data to runtime field

Allows it to be preserved during copy-on-write update when on-geometry
related update is needed.

This is a required part for T63537, where we need to preserve the entire
evaluation data when object is tagged for only RECALC_COPY_ON_WRITE.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5023

source/blender/modifiers/intern/MOD_particleinstance.c

@@ -415,7 +415,11 @@ static Mesh *applyModifier(ModifierData *md, const ModifierEvalContext *ctx, Mes
             ChildParticle *cpa = psys->child + (p - psys->totpart);
             pa = psys->particles + (between ? cpa->pa[0] : cpa->parent);
           }
-          psys_mat_hair_to_global(sim.ob, sim.psmd->mesh_final, sim.psys->part->from, pa, hairmat);
+          psys_mat_hair_to_global(sim.ob,
+                                  BKE_particle_modifier_mesh_final_get(sim.psmd),
+                                  sim.psys->part->from,
+                                  pa,
+                                  hairmat);
           copy_m3_m4(mat, hairmat);
           /* to quaternion */
           mat3_to_quat(frame, mat);