CGSize * scale and mid_{xy}

```ruby
size = CGSize.new(10, 10)
size * 2  # => CGSize.new(20, 20)
size / 2  # => CGSize.new(5, 5)

rect = CGRect.new([10, 10], [10, 10])
rect.mid_x  # => 5
rect.mid_x(true)  # => 15
```