Add a configuration directive for custom DPI to be used when converting SVG to PNG

[znerol]

CairoSVG allows to specify the DPI value when converting SVG to PNG. Let's introduce a configuration directive in order to allow users to control this value.

[sbaechler]

What is it supposed to do?

[znerol]

DPI = ratio between 1in and 1px, see help text of the cairosvg command line:

$ cairosvg --help
Usage: cairosvg filename [options]

Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  -f FORMAT, --format=FORMAT
                        output format
  -d DPI, --dpi=DPI     ratio between 1in and 1px
  -o OUTPUT, --output=OUTPUT
                        output filename

[sbaechler]

So would this only apply to SVGs that have their size unit set to 'in' or 'cm'?
Pretty much all SVGs that I came across had their size defined in pixels.

[znerol]

So would this only apply to SVGs that have their size unit set to 'in' or 'cm'?

Yes, indeed.

Pretty much all SVGs that I came across had their size defined in pixels.

Problem is that those which come in units other than pixels tend to look like crap if you render them with 96 DPI (which is the CairoSVG default).

[guilhermef]

@znerol Can you make a PR ?