Add miDrawPolylines and miFillPolylines.

Need a new Boolean attribute in the graphics context, indicating whether
polylines and polyarcs that repeat the initial point as the final point
should be regarded as closed.  The default value should be `true', which is
the way we now do it (and X11 does it).  But there should be support for
`false' too, since a modern Postscript-style path that repeats the initial
point as the final point is not necessarily closed.

Add support for computation of a bounding box?

Add support for clipping; to user-specified rectangular regions at least,
or perhaps to a user-specified union of rectangles?  Or clipping path /
polygon?

Add support for triangular line caps to miDrawLines().  Right now,
triangular line caps are supported for arcs only.
