Return the stipple pixmap name of FACE, or nil if unspecified.
If the optional argument FRAME is given, report on face FACE in that frame.
If FRAME is t, report on the defaults for face FACE (for new frames).
If FRAME is omitted or nil, use the selected frame.
If INHERIT is nil, only a stipple directly defined by FACE is
considered, so the return value may be nil.
If INHERIT is t, and FACE doesn't define a stipple, then any stipple
that FACE inherits through its `:inherit' attribute is considered as
well; however the return value may still be nil.
If INHERIT is a face or a list of faces, then it is used to try to
resolve an unspecified stipple.
To ensure that a valid stipple or nil is always returned, use a value of
`default' for INHERIT; this will resolve any unspecified values by merging
with the `default' face (which is always completely specified).