These attributes supply the fillrectclass object with an area fill
pattern.  The IA_APattern attribute points to the area fill pattern for
the object.  The IA_APatSize attribute is the depth of the area fill
pattern.  These attribute values are similar to the parameters passed to
the SetAfPt() macro (defined in <graphics/gfxmacros.h>) and indirectly
correspond to fields in a rastport structure.  for more information on
these patterns, see the section on patterns in the "graphics primitives"
chapter of the Amiga ROM Kernel Reference Manual: Libraries.