This call is similar to SET-FILL-STYLE, except that it sets
a user-defined pattern instead of a predefined one.
A fill-pattern is an 8x8 matrix used to fill surfaces;
at the PCSCHEMElevel, it is a list of 8-bit integers,
each coding one row.
Normally there will be 8 elements in the list but if you put less,
PCS will assume you want to repeat periodically the given ones.
For example, the chain pattern
(SET-FILL-PATTERN '(#b01000100
#b00111000
#b01000100
#b01000100
#b01000100
#b00111000
#b01000100
#b01000100)
)
will produce the same result as
(SET-FILL-PATTERN '(#b00111000
#b01000100
#b01000100
#b01000100)
)