feat(point): Intent to ship point.radialGradient

[netil]

Issue

#3405

Details

Implement point.radialGradient option

point: {
  // apply radial gradient with default value
  radialGradient: true,

  // or apply radial gradient with customized value
  radialGradient: {
     cx: 0.5,
     cy: 0.5,
     r: 0.5,
	stops: [
	  [0.3, "#fff", 0.8],
	  [0.6, function(id) { return id === "data1" ? this.color(id) : "green"; }, 0.35],
	  [1, null, 1]
	]
  }
}