Add MenuScene with New Level UI

* Debug mode: no longer enables the DebugOverlay (F3) by default, but
  does now insert the current FPS counter into the window title bar.
* ui.Frame: set a default "mostly transparent" BG color so the frame
  background doesn't render as white.
* Add the MenuScene which will house the game's main menus.
* The "New Level" menu is first to be added.
  * UI lets you pick Page Type and Wallpaper using radio buttons.
  * Page Type: Unbounded, Bounded (default), No Negative Space, Bordered
  * Fix bugs in uix.Canvas to fully support all these page types.

interface.go

@@ -22,6 +22,7 @@ type Engine interface {
 
 	// Clear the full canvas and set this color.
 	Clear(Color)
+	SetTitle(string)
 	DrawPoint(Color, Point)
 	DrawLine(Color, Point, Point)
 	DrawRect(Color, Rect)

sdl/sdl.go

@@ -84,6 +84,12 @@ func (r *Renderer) Setup() error {
 	return nil
 }
 
+// SetTitle sets the SDL window title.
+func (r *Renderer) SetTitle(title string) {
+	r.title = title
+	r.window.SetTitle(title)
+}
+
 // GetTicks gets SDL's current tick count.
 func (r *Renderer) GetTicks() uint32 {
 	return sdl.GetTicks()