Remove the per-zone mutex that serialized all layer fills of the
same zone. fillSingleZone() is read-only on zone state, so
concurrent fills of different layers are safe. Make m_needRefill
atomic and narrow the CacheTriangulation lock to the map lookup.
Also add an R-tree index to reduce queries over large zones.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/23450