Removed registering glfwTerminate with atexit.

Functions registered with atexit are called from the thread calling exit.
glfwTerminate should only be called from the main thread.  Mistakes should be
explicit.

examples/heightmap.c

@@ -595,6 +595,8 @@ int main(int argc, char** argv)
 
         free(vertex_shader_src);
         free(fragment_shader_src);
+
+        glfwTerminate();
         exit(EXIT_FAILURE);
     }
 
@@ -608,6 +610,8 @@ int main(int argc, char** argv)
         fprintf(stderr, "ERROR: unable to resolve OpenGL function pointers\n");
         free(vertex_shader_src);
         free(fragment_shader_src);
+
+        glfwTerminate();
         exit(EXIT_FAILURE);
     }
     /* Prepare opengl resources for rendering */
@@ -619,6 +623,8 @@ int main(int argc, char** argv)
     {
         fprintf(stderr, "ERROR: during creation of the shader program\n");
         usage();
+
+        glfwTerminate();
         exit(EXIT_FAILURE);
     }
 
@@ -683,6 +689,7 @@ int main(int argc, char** argv)
         }
     }
 
+    glfwTerminate();
     exit(EXIT_SUCCESS);
 }