Fix error caused by particle spawning during long frames.

src/Emitter.ts

@@ -785,7 +785,7 @@ export class Emitter
                                 particle.position.y += emitPosY;
 
                                 // also, just update the particle's age properties while we are looping through
-                                particle.age += delta;
+                                particle.age += -this._spawnTimer;
                                 // determine our interpolation value
                                 let lerp = particle.age * particle.oneOverLife;// lifetime / maxLife;