Small improvement to sprite renderer, added demo for simple sprite rendering.

demo/sprite/sprite.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"github.com/DeKugelschieber/go-game"
+)
+
+const (
+	gopher_path = "src/github.com/DeKugelschieber/go-game/demo/sprite/assets/gopher.png"
+)
+
+type Game struct{}
+
+func (g *Game) Setup() {
+	res, err := goga.LoadRes(gopher_path)
+
+	if err != nil {
+		panic(err)
+	}
+
+	tex, ok := res.(*goga.Tex)
+
+	if !ok {
+		panic("Resource is not a texture")
+	}
+
+	sprite := goga.NewSprite(tex)
+	renderer := goga.GetSystemByName("spriteRenderer")
+	renderer.Add(sprite)
+}
+
+func (g *Game) Update(delta float64) {
+}
+
+func main() {
+	game := Game{}
+	options := goga.RunOptions{ClearColor: goga.Vec4{1, 1, 1, 0},
+		Resizable:           true,
+		SetViewportOnResize: true,
+		ExitOnClose:         true}
+	goga.Run(&game, &options)
+}