message: Call cleanup hook every frame.
  Device::cleanup() function has to be called every frame to allow backend
  to release resources.
diff: @@ -16,6 +16,7 @@ use sdl2;
  use rusttype;
  use image;
  use gfx;
  +use gfx::traits::Device;
  use gfx::traits::FactoryExt;
  use gfx_device_gl;
  use gfx_window_sdl;
  @@ -349,6 +350,7 @@ pub fn present(ctx: &mut Context) {
  // thing is a bigger hurdle, so this is fine for now.
  gfx.encoder.flush(&mut *gfx.device);
  gfx.window.gl_swap_window();
  + gfx.device.cleanup();
  }
  pub fn arc(ctx: &mut Context,
  