When working with extracting multiple

Presently crop can be used by changing the viewport, But fitTo is still a load-time parameter.

This is the awkward method for crop before render (broken in 2.6.x; working in 2.5.x.
Per issue: #287

    const bbox = resvg.getBBox();
    bbox.x=rect.x;
    bbox.y=rect.y;
    bbox.width = rect.width;
    bbox.height = rect.height;
    resvg.cropByBBox(bbox)

Unfortunately, fitTo is only usable in the constructor:

    fitTo: {
      mode: 'height',
      value: 4096,
    },

When cutting out multiple areas of an SVG file and rendering to multiple sizes, it would be a bit nicer to be able to supply fitTo and crop into the render function itself, so the SVG is only loaded once but usable for many outputs.