Perform rendering on a separate thread

[johannesugb]

glfwSwapBuffers can be very very slow if there is input (e.g. mouse is moved) resulting in stuttering. As stated in [1], it may only be called from the main thread, but rendering can be performed from secondary threads (see [2]).

=> Move rendering to a secondary thread!

[1] http://www.glfw.org/docs/latest/group__window.html#ga37bd57223967b4211d60ca1a0bf3c832
[2] http://www.glfw.org/docs/3.0/group__context.html#ga15a5a1ee5b3c2ca6b15ca209a12efd14