Don't re-render subtrees who's size don't change on state update about litho

[appleguy]

@mihaelao curious if this optimization has been implemented, or is still future work.

I found it was difficult to implement this when using Yoga in the Texture framework, since the constraints used for items in the middle of the tree are not exposed outside of Yoga. However, with the flexbox-based ASLayoutSpec system in Texture, this optimization proved very effective.

In the latter case, the API contract is simply that parents must inform their children if constraints change. Until then, children can 1. re-measure with the same constraints, and 2. if the resulting size is the same, there is no need to invalidate the tree above.

[colriot]

Adding @adityasharat for the context. This is currently running under experiment and close to shipping.