Adds ability to define tile sub-cells in mod.yaml

Implements #6086 - Number and offsets of SubCells should not be hardcoded.
IMO an offset restriction is redundant.  2048 is a valid offset from a code perspective (nothing is going to crash), but bogus from a gameplay perspective (cells are by definition 1024 units wide).  If a modder wants to set a value outside this then it is their own fault that the gameplay will suffer.