title: How to pass String as GLchar** (char**) parameter in glShaderSource?
body: <p>I have shader that stored in a String value:</p>
  
  <pre><code>    var myShader = 
      "    attribute vec4 a_position;" +
      "    void main() {" +
      "    gl_Position = a_position;" +
      "    }"
  
      glShaderSource(shader, GLsizei(1), myShader, nil)
  </code></pre>
  
  <p>The function glShaderSource has the bind signature:</p>
  
  <pre><code>    func glShaderSource(shader: GLuint, count: GLsizei, string: UnsafePointer&lt;UnsafePointer&lt;GLchar&gt;&gt;, length: UnsafePointer&lt;GLint&gt;)
  </code></pre>
  
  <p>When I try to pass the shader String to glShaderSource directly I get the error message:</p>
  
  <pre><code>    'String' is not convertible to 'UnsafePointer&lt;UnsafePointer&lt;GLchar&gt;&gt;'
  </code></pre>
  
  <p>How to pass String correctly? (xCode Version 6.1 (6A1052d))</p>