DistanceProxy doesn't enforce convexity

[louis-langholtz]

Expected/Desired Behavior or Experience:

GetChild(const Shape& shape, ChildCounter index) returns a type that enforces the convexity requirement. Enforcement has a lower up-front computational burden than having to build an envelope on construction of the type that gets returned.

Actual Behavior:

GetChild(const Shape& shape, ChildCounter index) returns a DistanceProxy which doesn't enforce convexity.

Possible Solution:

Return an iterator type which enforces the convexity invariant on updating to the next location perhaps using a dot product calculation.

The Goal:

Besides impacting performance as little as possible, the solution would have as little memory overhead as possible so that a given shape can efficiently generate the vertices of a child on the fly.