`RsvgLength` values are used in [method@Rsvg.
Handle.get_intrinsic_dimensions], for example, to return the CSS length values of the `width` and `height` attributes of an `<svg>` element.
This is equivalent to CSS lengths.
It is up to the calling application to convert lengths in non-pixel units (i.e. those where the unit field is not
`RSVG_UNIT_PX`) into something meaningful to the application. For example, if your application knows the dots-per-inch (DPI) it is using,
it can convert lengths with unit in `RSVG_UNIT_IN` or other physical units.