Refactor ParallaxComponent (#613)

* Simplified loading of ParallaxComponent

* Loading helpers for the different Parallax parts

And refactor how the delta velocity works

* Fix formatting

* Break out Parallax out of ParallaxComponent

* Fix docs

* Add extension for loading different parallax things on game

* Fix formatting

* Add loadParallaxComponent extension

* Fix formatting

doc/examples/parallax/lib/main_advanced.dart

@@ -0,0 +1,43 @@
+import 'package:flame/flame.dart';
+import 'package:flame/parallax.dart';
+import 'package:flame/components/parallax_component.dart';
+import 'package:flame/extensions/vector2.dart';
+import 'package:flame/game.dart';
+import 'package:flutter/material.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Flame.util.fullScreen();
+  runApp(
+    GameWidget(
+      game: MyGame(),
+    ),
+  );
+}
+
+class MyGame extends BaseGame {
+  final _layersMeta = {
+    'bg.png': 1.0,
+    'mountain-far.png': 1.5,
+    'mountains.png': 2.3,
+    'trees.png': 3.8,
+    'foreground-trees.png': 6.6,
+  };
+
+  @override
+  Future<void> onLoad() async {
+    final layers = _layersMeta.entries.map(
+      (e) => loadParallaxLayer(
+        e.key,
+        velocityMultiplier: Vector2(e.value, 1.0),
+      ),
+    );
+    final parallax = ParallaxComponent(
+      Parallax(
+        await Future.wait(layers),
+        baseVelocity: Vector2(20, 0),
+      ),
+    );
+    add(parallax);
+  }
+}