Add support for blend modes

[timbtimbtimb]

It would be fantastic if CairoSVG could support CSS blend modes

<svg viewBox="0 0 120 120"> 
  <g>
    <circle style="mix-blend-mode: screen;" cx="40" cy="40" r="40" fill="red" />
    <circle style="mix-blend-mode: screen;" cx="80" cy="40" r="40" fill="lightgreen" />
    <circle style="mix-blend-mode: screen;" cx="60" cy="80" r="40" fill="blue" />
  </g>
</svg>