Fix actualBoundingBoxLeft/Right with center/right alignment

[zbjornson]

This bug goes back 10 years to the original implementation.

Fixes #1909

• Have you updated CHANGELOG.md?

[stepancar]

@zbjornson , @chearon , it seems like after this change actualBoundingBoxLeft was broken for textAlign = start.

Because it returns value with opposite sign comparing to browser

[zbjornson]

@stepancar can you provide a repro in a new issue please? It seems to be correct:

tests['measureText()'] = function (ctx) {
  function drawWithBBox (text, x, y) {
    ctx.fillText(text, x, y)
    ctx.strokeStyle = 'red'
    ctx.beginPath(); ctx.moveTo(0, y + 0.5); ctx.lineTo(200, y + 0.5); ctx.stroke()
    const metrics = ctx.measureText(text)
    ctx.strokeStyle = 'blue'
    ctx.strokeRect(
      // positive numbers for actualBoundingBoxLeft indicate a distance going left
      x + metrics.actualBoundingBoxLeft + 0.5,
      y - metrics.actualBoundingBoxAscent + 0.5,
      metrics.width,
      metrics.actualBoundingBoxAscent + metrics.actualBoundingBoxDescent
    )
  }

  ctx.font = '20px Arial'
  ctx.textBaseline = 'alphabetic'
  ctx.textAlign = 'start'
  drawWithBBox('start', 10, 150)
}