Add initial "event loop" and scene

Start with the main menu with only one option, and build the
abstractions between the event loop and what the scene itself handles.

lib/lib.go

@@ -1,22 +1,98 @@
 package lib
 
-import "github.com/hajimehoshi/ebiten/v2"
+import (
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+	"github.com/hajimehoshi/ebiten/v2/inpututil"
+)
+
+// Scenes: Main Menu
+// Game Mode
+
+//Scene - is the highest-level element of state the game can be in.
+
+type GameEvent struct {
+	Type int
+}
+
+type HandleResult int
+
+const (
+	EventNoOp int = iota
+	EventKeyRelease
+)
+
+const (
+	DoNothing HandleResult = iota
+	EndProgram
+)
+
+type Scene interface {
+	HandleEvent(*GameEvent) HandleResult
+	Draw(screen *ebiten.Image)
+}
 
 // Implements ebiten.Game
 type TetrisGame struct {
+	activeScene Scene
+}
+
+type MainMenuScene struct {
+	selections            [1]string
+	currentSelectionIndex int
+}
+
+func (menu *MainMenuScene) HandleEvent(e *GameEvent) HandleResult {
+	if e.Type == 1 {
+		if menu.selections[menu.currentSelectionIndex] == "Quit" {
+			return EndProgram
+		}
+	}
+	return DoNothing
+}
+
+func (menu *MainMenuScene) Draw(screen *ebiten.Image) {
+	ebitenutil.DebugPrint(screen, menu.selections[menu.currentSelectionIndex])
 }
 
 func NewGame() (*TetrisGame, error) {
-	return &TetrisGame{}, nil
+	return &TetrisGame{
+		activeScene: &MainMenuScene{
+			selections:            [1]string{"Quit"},
+			currentSelectionIndex: 0,
+		},
+	}, nil
 }
 
 func (g *TetrisGame) Layout(outsideWidth, outsideHeight int) (int, int) {
+	//TODO: Make this do whatever it is supposed to do : resizing etc
 	return 640, 480
 }
 
 func (g *TetrisGame) Update() error {
+	//while there are events
+
+	var keys []ebiten.Key
+	keys = inpututil.AppendJustReleasedKeys(keys)
+
+	var event GameEvent
+	if len(keys) > 0 {
+		event = GameEvent{Type: EventKeyRelease}
+	} else {
+		event = GameEvent{Type: EventNoOp}
+	}
+
+	switch g.activeScene.HandleEvent(&event) {
+	case EndProgram:
+		return ebiten.Termination
+	case DoNothing:
+		return nil
+	default:
+		return nil
+	}
 	return nil
 }
 
 func (g *TetrisGame) Draw(screen *ebiten.Image) {
+	g.activeScene.Draw(screen)
 }