title: add new data to a buffer while keeping existing data in OpenGL
body: <p>I want to add new data to my buffer.</p>
  
  <p>I can't use <code>glBufferSubData</code> since the buffer does not have enough allocated memory.</p>
  
  <p>When I try to change the buffer size with <code>glBufferData</code> beforehand, the already existing data gets lost.</p>
  
  <p>What's the correct way to add new data while keeping existing data?</p>