A TileRun represents a single run of the tiling software. It is a
logical unit of geometrical information that consists of a set of
TilingMasks, and Tiles
that are associated with exactly one TileRun (ie each TilingBoundary,
TilingMask, and Tile is associated with exactly one TileRun, with one
exception that will be noted later). Sometimes this is also called a tiling
chunk, although that is incorrect usage of the term and it should not be
confused with an imaging chunk.