Comment by bnieuwen

So there are two parts to this: the app producing the image in the application window and then the windowing system combining multiple windows together to form the final image you see on screen.

The former gets done in process (using e.g. GL/vulkan) and then that final image gets passed onto the windowing system which is a separate process and could run outside the container.

As an aside, with accelerated graphics you mostly pass a file descriptor to the GPU memory containing the image, rather than mucking around with traditional shared memory.