PPos is best thought of as a cell grid applied in
screen space. Multiple cells with different
terrain heights may be projected to the same PPos,
or to multiple PPos if they do not align with the
screen grid.
PPos coordinates are used primarily for map edge
checks and shroud / visibility queries.
To resolve the ambiguity introduced when the introduction of isometric maps meant that cell and map coordinates were no longer equivalent, a new type has been introduced so they can each be represented separately.
