title: How to convert Vec<Rgb<u8>> to Vec<u8>
body: <p>Using the Piston <code>image</code> crate, I can write an image by feeding it a <code>Vec&lt;u8&gt;</code>, but my actual data is <code>Vec&lt;Rgb&lt;u8&gt;&gt;</code> (because that is a lot easier to deal with, and I want to grow it dynamically).</p>
  
  <p>How can I convert <code>Vec&lt;Rgb&lt;u8&gt;&gt;</code> to <code>Vec&lt;u8&gt;</code>? <code>Rgb&lt;u8&gt;</code> is really <code>[u8; 3]</code>. Does this have to be an <code>unsafe</code> conversion?</p>