add multiscene demo

demo/multiscene/.gitignore

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+.dub
+docs.json
+__dummy.html
+docs/
+/multiscene
+multiscene.so
+multiscene.dylib
+multiscene.dll
+multiscene.a
+multiscene.lib
+multiscene-test-*
+*.exe
+*.o
+*.obj
+*.lst

demo/multiscene/dub.sdl

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+name "multiscene"
+description "multiple scenes"
+authors "redthing1"
+copyright "Copyright © 2020, redthing1"
+license "proprietary"
+dependency "reng" path="../.."
+
+subConfiguration "reng" "lib-minimal"

demo/multiscene/source/app.d

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+import std.stdio;
+
+import re;
+import re.math;
+import std.stdio;
+import play;
+
+class Game : Core {
+	enum WIDTH = 800;
+	enum HEIGHT = 400;
+
+	this() {
+		super(WIDTH, HEIGHT, "multiscene");
+	}
+
+	override void initialize() {
+		default_resolution = Vector2(WIDTH / 4, HEIGHT / 4);
+		content.paths ~= "../content/";
+
+		load_scenes([new PlayScene()]);
+	}
+}
+
+void main() {
+	auto game = new Game(); // init game
+	game.run();
+	game.destroy(); // clean up
+}

demo/multiscene/source/play.d

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+module play;
+
+import re;
+import re.gfx;
+import re.gfx.shapes.rect;
+import re.ng.camera;
+import re.math;
+import re.phys.kin2d;
+
+static import raylib;
+
+class PlayScene : Scene2D {
+    override void on_start() {
+        resolution = Vector2(100, 100);
+        output_rect = Rectangle(
+            0, 0,
+            Core.window.screen_width / 2,
+            Core.window.screen_height
+        );
+
+        auto bg_tween = Tweener.tween(clear_color, Colors.DARKGRAY,
+            Colors.LIGHTGRAY, 2, &Ease.QuadIn);
+        bg_tween.start();
+
+        auto box1 = create_entity("box1", Vector2(20, 20));
+        box1.add_component(new ColorRect(Vector2(8, 8), Colors.BLUE));
+        auto box1_body = box1.add_component!KinBody2D();
+        box1_body.angular_accel = 0.1;
+
+        auto box2 = create_entity("box2", Vector2(40, 20));
+        box2.add_component(new ColorRect(Vector2(8, 8), Colors.RED));
+        auto box2_body = box2.add_component!KinBody2D();
+        box2_body.angular_accel = -0.07;
+
+        // follow the first box
+        cam.entity.add_component(new CameraFollow2D(box1, 0.05));
+    }
+}