Instances of this class will make drawing operations (stroking,
filling) to become partially transparent. value defines the
transparency factor in the range 0 (opaque) to 1
(transparent).
Transparency is available in PDF output only since it is not
supported by PostScript. However, for certain ghostscript devices
(for example the pdf backend as used by ps2pdf) proprietary
PostScript extension allows for transparency in PostScript code too.
PyX creates such PostScript proprietary code, but issues a
warning when doing so.
