Fix main loop, add example and cleanup

samples/button.rb

@@ -1,4 +1,4 @@
-app = App.new 400, 400, title: "Button Test"
+App.run 400, 400, title: "Button Test"
 
 main_scene = Scene.new
 
@@ -15,7 +15,11 @@ rect.on_click do
     0xFF0000
 end
 
+main_scene.update do |dt|
+  puts "Delta time: #{dt}ms"
+end
+
 main_scene << text
 main_scene << rect
 
-app.start main_scene
+App.start main_scene

samples/main.rb

@@ -5,15 +5,15 @@ width, height = 720, 480
 
 image = Tiles.from("assets/loading.png").first
 
-app = App.new width, height, title: "My Game", loading_bg: image # can be image or animation
+App.run width, height, title: "My Game", loading_bg: image # can be image or animation
 
-app.map_key ?w, :forward
-app.map_key ?s, :backward
-app.map_key ?a, :left
-app.map_key ?d, :right
+App.map_key ?w, :forward
+App.map_key ?s, :backward
+App.map_key ?a, :left
+App.map_key ?d, :right
 
-app.map_key ?h, :heal
-app.map_key ?t, :place
+App.map_key ?h, :heal
+App.map_key ?t, :place
 
 menu_scene = Scene.new
 
@@ -24,7 +24,7 @@ start_button = ElementText.new "Start Game", font: default_font, position: {x: 1
 game_scene = Scene.new
 
 start_button.on_click do
-  app.start game_scene
+  App.start game_scene
 end
 
 menu_scene << start_button
@@ -184,4 +184,4 @@ world.shader :fog do |shader|
   z = 100 # shaders always appear above normal sprites but z is used to sort them relative to each other, so a shader with z 100 will appear above a shader with z 50, but both will appear above normal sprites
 end
 
-app.start menu_scene # first scene to show when the game starts, stops the loading bg
+App.start menu_scene # first scene to show when the game starts, stops the loading bg