examples: add fireworks example (#8358)

examples/fireworks/fireworks.v

@@ -0,0 +1,70 @@
+import objects
+import gg
+import gx
+import rand
+
+struct App {
+mut:
+	gg      &gg.Context = 0
+	rockets []objects.Rocket
+	frames  [][]objects.Rocket
+	// i thought about using a fixed fifo queue for the frames but the array 
+	// seemed to work fine, if you'd like a challenge try implementing it with the queue :)
+}
+
+fn on_frame(mut app App) {
+	app.gg.begin()
+
+	// drawing previous frames
+	for mut frame in app.frames {
+		for mut rocket in frame {
+			if !rocket.exploded {
+				rocket.color.a = byte(f32_max(rocket.color.a - 8, 0))
+				rocket.draw(mut app.gg)
+			}
+		}
+	}
+
+	// chance of firing new rocket
+	if rand.intn(30) == 0 {
+		app.rockets << objects.new_rocket()
+	}
+	// simulating rockets
+	app.rockets = app.rockets.filter(!it.dead)
+
+	for mut rocket in app.rockets {
+		rocket.tick(mut app.gg)
+	}
+
+	// adding frame
+	mut frame := app.rockets.clone()
+
+	for mut rocket in frame {
+		rocket.particles = []
+	}
+
+	app.frames << frame
+
+	// trimming out frames
+	if app.frames.len > 30 {
+		app.frames.delete(0)
+	}
+
+	app.gg.end()
+}
+
+fn main() {
+	mut app := &App{}
+
+	app.gg = gg.new_context(
+		width: objects.width
+		height: objects.height
+		window_title: 'Fireworks!'
+		bg_color: gx.black
+		use_ortho: true
+		user_data: app
+		frame_fn: on_frame
+	)
+
+	app.gg.run()
+}