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.

src/init.c

@@ -127,8 +127,6 @@ GLFWAPI int glfwInit(void)
         return GL_FALSE;
     }
 
-    atexit(glfwTerminate);
-
     _glfwInitialized = GL_TRUE;
 
     // Not all window hints have zero as their default value