Adds better explosions, shield, and game over

sky/sdk/example/game/lib/particle_system.dart

 part of sprites;
 
 class _Particle {
   Vector2 pos;
   Vector2 startPos;
 
   double colorPos;
   double deltaColorPos;
 
   double size;
@@ skipping 265 matching lines @@
   }
 
   double randMinus1To1() => _rand.nextDouble() * 2.0 - 1.0;
 }
 
 // TODO: Needs bindings to Skia method in SkCanvas (exclude canvas parameter)
 void drawAtlas(Canvas canvas, Image image, List<RSTransform> transforms, List<Rect> rects, List<Color> colors,
                TransferMode transferMode, Paint paint) {
   assert(transforms.length == rects.length && transforms.length == colors.length);
 
-  Paint paint = new Paint();
-
   Texture mainTexture = new Texture(image);
 
   for (int i = 0; i < transforms.length; i++) {
     RSTransform transform = transforms[i];
     Rect rect = rects[i];
     Color color = colors[i];
 
     canvas.save();
 
     Matrix4 matrix = new Matrix4(transform.scos, transform.ssin, 0.0, 0.0,
@@ skipping 14 matching lines @@
 
 // TODO: Needs bindings to Skia SkRSXform
 class RSTransform {
   double scos;
   double ssin;
   double tx;
   double ty;
 
   RSTransform(this.scos, this.ssin, this.tx, this.ty);
 }