视频播放列表更新，增加分享海报生成

gcanvas/context-2d/FillStyleRadialGradient.js

@@ -0,0 +1,17 @@
+class FillStyleRadialGradient {
+    constructor(x0, y0, r0, x1, y1, r1) {
+        this._start_pos = { _x: x0, _y: y0, _r: r0 };
+        this._end_pos = { _x: x1, _y: y1, _r: r1 };
+        this._stop_count = 0;
+        this._stops = [0, 0, 0, 0, 0];
+    }
+
+    addColorStop(pos, color) {
+        if (this._stop_count < 5 && 0.0 <= pos && pos <= 1.0) {
+            this._stops[this._stop_count] = { _pos: pos, _color: color };
+            this._stop_count++;
+        }
+    }
+}
+
+export default FillStyleRadialGradient;