Better commenting throughout

internal/world/surface.go

@@ -7,6 +7,8 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/vector"
 )
 
+// Surface types
+
 type SurfaceTag int
 
 const (
@@ -67,6 +69,8 @@ func (s *Surface) Draw(screen *ebiten.Image) {
 	)
 }
 
+// World container
+
 type World struct {
 	Surfaces []*Surface
 }