add placeholder for viewer tool

tools/viewer/main.go

@@ -0,0 +1,39 @@
+package main
+
+import "github.com/veandco/go-sdl2/sdl"
+
+func main() {
+	if err := sdl.Init(sdl.INIT_EVERYTHING); err != nil {
+		panic(err)
+	}
+	defer sdl.Quit()
+
+	window, err := sdl.CreateWindow("test", sdl.WINDOWPOS_UNDEFINED, sdl.WINDOWPOS_UNDEFINED,
+		800, 600, sdl.WINDOW_SHOWN)
+	if err != nil {
+		panic(err)
+	}
+	defer window.Destroy()
+
+	surface, err := window.GetSurface()
+	if err != nil {
+		panic(err)
+	}
+	surface.FillRect(nil, 0)
+
+	rect := sdl.Rect{X: 0, Y: 0, W: 200, H: 200}
+	surface.FillRect(&rect, 0xffff0000)
+	window.UpdateSurface()
+
+	running := true
+	for running {
+		for event := sdl.PollEvent(); event != nil; event = sdl.PollEvent() {
+			switch event.(type) {
+			case *sdl.QuitEvent:
+				println("Quit")
+				running = false
+				break
+			}
+		}
+	}
+}