dviread: usetex for the PDF backend

Hi,

I have continued my work on supporting text.usetex for the PDF backend
by parsing dvi files. It "sort of" works now, with the following
caveats:

- Your TeX distribution must have a kpsewhich command and a correctly
   configured pdftex.map file. This is true for modern distributions on
   Unix; I have no idea about Windows.
- TeX "virtual fonts" are not supported yet; Computer Modern should
   work, but Times will usually not. Truetype fonts also are not yet
   supported (for usetex; they are for ordinary text).
- Re-encoded fonts (in pdftex.map) probably do not work properly, and
   "font effects" such as slanting do not work.
- Bounding boxes are not quite right.
- The embedded Type 1 fonts are not subsetted, so output files are
   unnecessarily big.
- The resulting PDF files sometimes crash Preview.app on OS X, and even
   if it doesn't crash, it doesn't show the fonts. There is probably
   some problem in the embedding, but xpdf, gv, and Adobe Reader seem to
   show the files just fine.

To enable, rename _draw_tex to draw_tex in backend_pdf.py and set
text.usetex to True.

···

--
Jouni K. Sepp�nen
http://www.iki.fi/jks