Update to use new sdf-2d API

src/helper/circle.ts

@@ -0,0 +1,13 @@
+import { vec2 } from 'gl-matrix';
+
+export class Circle {
+  constructor(public center: vec2, public radius: number) {}
+
+  public distance(target: vec2): number {
+    return vec2.distance(this.center, target) - this.radius;
+  }
+
+  public distanceBetween(target: Circle): number {
+    return vec2.distance(target.center, this.center) - this.radius - target.radius;
+  }
+}