[Patch]: Added support for SVG graphics with dot

Hello,

I've created a patch which adds support for generating Scaleable Vector Graphics instead 
of bitmaps (for HTML output).

Most today's browsers have at least limited support für rendering svg graphics. I got the 
best results with Opera, Konqueror is nearly Ok, Firefox has some work left...

SVG graphics are included with the <object></object> tag instead of <img>. I got most 
information from SELFHTML (german only):

http://de.selfhtml.org/html/multimedia/objekte.htm#datendateien

For browsers which doens't support rendering SVG it's possible to show an alternative 
content (e.g. a png graphic). Thus my patch creates additional png (fixed) graphics when 
the user selects svg.


Patch description (generated with 'svn diff'):
- config.l:
  o Added 'svg' item for DOT_IMAGE_FORMAT

- dot.h
  o Added prototype for getPngImageSize() (see open issues)

- dot.cpp
  o Implemented getPngImageSize() (see open issues)
  o Generate an additional png file if the user selected svg format
  o Include both files as mentioned in the above tutorial

- htmlvisitor.cpp:
  o Generate an additional png file if the user selected svg format
  o Include both files as mentioned in the above tutorial

I've tried to adopt the existing coding style as good as possible. There should be no 
additional changes than those mentioned above. If the user doesn't select the svg format, 
the patch shouldn't have any effects.


Open issues:
- Image size of the SVG graphics
It's not very straightforward to generate an svg file with the same image size compared to 
an equal bitmap file. For reasons I don't understand the dot tool uses different sizes for 
svg and png graphics. I've tried to change this by setting DPI to 96 in dot but the resulting 
graphic was not rendered correctly in most of my browsers.

So I've decided to get the destination size from the png file (which is also created) and to 
scale the svg to this value. This possibly strange way produced the most acceptable 
results.

- Hyperlinks in the SVG graphics
For resons I don't know (perhaps a bug), all hyperlinks in the doxygen generated dot files 
start with a '$' character. This char is also present in the generated map file. I've never 
seen this before but it seems that it doesn't harm.
For svg graphics at least some browsers don't use the map file, they get information about 
hyperlinks directly from the svg file. Dot also includes the $ from the dot file here. At least 
in this case, my browsers failed to follow the link until I removed the $ char. Maybe it 
would be the best to remove it at all.


regards
Christian

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