Creates an opacity keyframe animation rule and returns its name.
Since most mobile Webkits have timing issues with animation-delay,
we create separate rules for each line/segment.
function addAnimation(alpha, trail, i, lines) {
    var name = ['opacity', trail, ~~(alpha*100), i, lines].join('-')
      , start = 0.01 + i/lines*100
      , z = Math.max(1 - (1-alpha) / trail * (100-start), alpha)
      , prefix = useCssAnimations.substring(0, useCssAnimations.indexOf('Animation')).toLowerCase()
      , pre = prefix && '-'+prefix+'-' || ''

    if (!animations[name]) {
      sheet.insertRule(
        '@' + pre + 'keyframes ' + name + '{' +
        '0%{opacity:' + z + '}' +
        start + '%{opacity:' + alpha + '}' +
        (start+0.01) + '%{opacity:1}' +
        (start+trail) % 100 + '%{opacity:' + alpha + '}' +
        '100%{opacity:' + z + '}' +
        '}', sheet.cssRules.length)

      animations[name] = 1
    }
    return name
  }