Some updates

buffer.go

@@ -32,8 +32,8 @@ type Buffer struct {
 	minX, minY int
 }
 
-func NewBuffer() Buffer {
-	return Buffer{cells: make(map[Coord]Cell)}
+func NewBuffer() *Buffer {
+	return &Buffer{cells: make(map[Coord]Cell)}
 }
 
 func (b *Buffer) Width() int  { return b.maxX - b.minX }