Add MG_OBJPILE flag, which windowports can use to check if a location
has more than one object stack. If use_inverse is on, TTY will use
inverse to show such piles. If a boulder is the topmost item on a pile,
then the object pile flag is not used; mainly because boulders are "solid",
boulders dropped by monsters are nearly always over other objects, and so
that special levels such a Sokoban can "hide" items under the boulders.
TODO: a "pilemark", analogous to "petmark", perhaps a green plus sign,
which can be used by windowports with tiles.
174e6f47b3