arc bounding box broken

[gabelerner]

make https://github.com/canvg/canvg/blob/master/src/canvg.js#L1738 better

test case is:

<svg width="300px" height="200px">
  <defs>
    <filter id="testfilter">
      <feColorMatrix type="saturate" values="0"></feColorMatrix>
    </filter>
  </defs>
  <g fill="#67b7dc" transform="translate(150,100)" filter="url('#testfilter')">
    <path d="M0,-71 a80,71,0,0,1,0,142 a80,71,0,0,1,0,-142" />
  </g>
</svg>

returns width of 0 but should be a bigger bounding box ... maybe add halfway point to it