title: glm::lookAt returns matrix with nan elements
body: <p>I want to create a view matrix for a camera which perpendicularly look at the ground:</p>
  
  <pre><code>glm::mat4 matrix = glm::lookAt(glm::vec3(0.0f, 1.0f, 0.0f), glm::vec3(0.0f, 0.0f, 0.0f), glm::vec3(0.0f, 1.0f, 0.0f));
  </code></pre>
  
  <p>The last argument is the global up vector so everything seems to be correct but I get following matirx:</p>
  
  <pre><code>-nan    -nan    -0  0   
  -nan    -nan     1  0   
  -nan    -nan    -0  0   
   nan     nan    -1  1
  </code></pre>
  
  <p>I guess that I get nan because a look at vector is parallel to up vector, but how can I build a correct view matrix using glm::lookAt function.</p>