DEG_add_collision_relations was only adding relationships for objects in
the brush collection, however dynamic paint also updates parent objects
and armature poses which weren't accounted for. Failing to include these
relations meant the evaluating the depsgraph could evaluate a pose
object from multiple threads at once - causing a crash.
Resolve by sharing logic between DEG_add_collision_relations &
BKE_object_modifier_update_subframe so depsgraph relationships match
objects the dynamic-paint modifier updates as part of its evaluation.
Ref !144844
fc7141f582