SvgSurface path should be an Option

[bilelmoussaoui]

Per the docs in https://www.cairographics.org/manual/cairo-SVG-Surfaces.html the filename param of cairo::SvgSurface::new should be optional see

filename | a filename for the SVG output (must be writable), NULL may be used to specify no output. This will generate a SVG surface that may be queried and used as a source, without generating a temporary file.

[sdroege]

Sounds like a good idea, do you want to provide a PR?