Fix marker-start being drawn on mid vertices

[rianmcguire]

Expected behaviour (Firefox 91):

WeasyPrint 53.2:

WeasyPrint with fix:

Example:

<svg viewBox="0 0 200 400" width="200">
  <g>
    <defs>
      <marker
        id="arrow"
        viewBox="0 0 10 10"
        refX="0"
        refY="5"
        markerWidth="4"
        markerHeight="4"
        orient="auto"
        fill="red"
        stroke="none"
      >
        <path d="M 0 0 L 10 5 L 0 10 z"></path>
      </marker>
    </defs>
    <path
      d="M 10 100 l 50, 10 l 50 0 l 50 -20"
      fill="none"
      stroke="black"
      stroke-width="2"
      marker-start="url(#arrow)"
    />
    <path
      d="M 10 200 l 50, 10 l 50 0 l 50 -20"
      fill="none"
      stroke="black"
      stroke-width="2"
      marker-end="url(#arrow)"
    />
    <path
      d="M 10 300 l 50, 10 l 50 0 l 50 -20"
      fill="none"
      stroke="black"
      stroke-width="2"
      marker-mid="url(#arrow)"
    />
  </g>
</svg>

[grewn0uille]

Thank you!