title: What values should a glm::mat4 made via a zero argument constructor contain?
body: <p>What values should a glm::mat4 made via a zero argument constructor contain? On 64-bit Windows 10, using 64-bit GLM v0.9.9.0 installed via vcpkg, the result of <code>glm::mat4()</code> is a 4x4 matrix filled with zeros. This is the same on 64-bit Ubuntu 18.04 LTS with default GLM.</p>
  
  <p>On the other hand, I can see near the top of GLM's <code>type_mat4x4.inl</code> that a definition exists which sets the contents equal to the identity matrix. (This is conditionally excluded on the two builds I describe above.) My colleague though has informed me that a call to <code>glm::mat4()</code> on his system does produce an identity matrix.</p>
  
  <p>Do such differences reflect a recent change in GLM? That is, would the differences disappear if we all used the latest version of GLM? Or, is it by design that GLM would produce two different results on two different systems?</p>