fix(runtime): fail to identify annotation as mark (close: #4106)

[pearmini]

See #4106 for more detail.

G2.render({
  type: 'layer',
  children: [
    {
      type: 'interval',
      coordinate: [{ type: 'theta', innerRadius: 0.5 }],
      scale: { color: { guide: null } },
      data: [
        { genre: 'Sports', sold: 275 },
        { genre: 'Strategy', sold: 115 },
        { genre: 'Action', sold: 120 },
        { genre: 'Shooter', sold: 350 },
        { genre: 'Other', sold: 150 },
      ],
      encode: {
        y: 'sold',
        color: 'genre',
      },
    },
    {
      type: 'annotation.text',
      data: [{ x: 0.5, y: 0.5, text: 'G2' }],
      scale: {
        x: { guide: null },
        y: { guide: null },
      },
      encode: {
        x: 'x',
        y: 'y',
        text: 'text',
        color: 'black',
      },
      style: {
        fontSize: 60,
        textBaseline: 'middle',
        fontWeight: 'bold',
      },
    },
  ],
})