As all other constraints, Child Of applies its effect on top of the

result of evaluating the preceeding stack. However, in case of this

constraint it makes it very cumbersome to switch between multiple

parents, as you have to either manually disable the previous Child Of

constraints at the instant of switch, or deal with weird effects

due to all parents affecting the object.

This adds a new constraint option to evaluate a constraint against

the transform at the start of the constraint stack. Of course, the

best solution for this is implementing node-based constraints, but

I think that this option to simply bypass all of the preceeding

stack is simple enough to add now.

Due to more complicated evaluation process, it becomes even harder

to correctly compute the inverse matrix from the outside, so this

also changes the Set Inverse operator to simply setting a flag for

the matrix to be recomputed during evaluation. This is similar to

how many other constraints with 'neutral state' data work.