Flag to enable/disable wrapping in another SVG

[vfosnar]

Gecko (not sure abt. chromium) requires both width and height attributes on the root SVG element to render it to a canvas.

These are missing by default in the wrapping container to allow scaling AFAIK.

Add a flag that removes the wrapping SVG container to allow external programs to render the SVG to a canvas.

[gavin-ts]

I think this may be fixed by running d2 with --scale=1 which should include the width and height on the outer SVG

[vfosnar]

Thank you<3 that works! Dunno why I missed the flag.