vg/vgsvg: consider adding the ability to embed fonts in SVG

[sbinet]

as evocated in #702, we could perhaps add an option to vg/vgsvg.Canvas (like we did for vg/vgpdf.Canvas) to embed fonts inside the SVG document.

the default should probably be to not embed fonts.

ex:

<svg xmlns="http://www.w3.org/2000/svg" width="450" height="150" font-size="24" text-anchor="middle">
    <defs>
        <style>
            @font-face{
                font-family:"Roboto Condensed";
                src:url(data:font/ttf;charset=utf-8;base64,your_base64_encoded_long_string) format("ttf"); 
                font-weight:normal;font-style:normal;
            }
            @font-face{
                font-family:"Open Sans";
                src:url(data:font/ttf;charset=utf-8;base64,your_base64_encoded_long_string) format("ttf"); 
                font-weight:normal;font-style:normal;
            }
            @font-face{
                font-family:"Anonymous Pro";
                src:url(data:font/ttf;charset=utf-8;base64,your_base64_encoded_long_string) format("ttf"); 
                font-weight:normal;font-style:normal;
            }
        </style>
    </defs>
        <text font-family="Roboto Condensed" x="190" y="32.92">
        This is a Roboto Condensed font
    </text>
    <text font-family="Open Sans" x="190" y="82.92">
        This is a Open Sans font
    </text>
    <text font-family="Anonymous Pro" x="190" y="132.92">
        This is a Anonymous Pro font
    </text>
</svg>

[kortschak]

SGTM