layout: Make incremental reflow more fine-grained by introducing "reflow out-of-flow" and "reconstruct flow" damage bits.

This is the last PR and most of the work for the maze solver and RoboHornet. 

r? @glennw 
cc @cgaebel 
Why is this one flag so far away from the others? Now we have 3 places where we leave random incremental reflow flags. :(