Cache the initial path layer for faster path search startups.

Reinitializing the initial cell info layer for the path search took a fair bit of time. We cache this initial setup so it only has to be done each time the map size changes. A CopyValuesFrom method in CellLayer is provided which copies values between layers by just copying the internal arrays for super speed.

This speeds up InitCellInfo 10x.
Merge pull request #7108 from RoosterDragon/faster-path-init

Cache the initial path layer for faster path search startups.