Support SVG marker-mid property

[hermet]

While 'marker-mid' is not included in the SVG Tiny 1.2 specification, there are practical use cases where it can be beneficial.

IN:

OUT:

sample: https://raw.githubusercontent.com/lipis/flag-icons/main/flags/4x3/us.svg doc: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/marker-mid history: https://github.com/thorvg/thorvg/issues/1451