[repacker] Improve vertex priority packing. Previous priority implementation would move a node further back within it's layer, but at max priority was unable to move any further up than that. This updates the implementation to have 3 priority levels: 1. Distance is reduced by half of table size. 2. Distance is reduced by full table size (move to beginning of the layer). 3. Distance is set to 0. Vertex will be packed as soon as possible. Also makes the iterative resolutions aware of max priority, so it won't keep trying to raise priority beyond the maximum.