Asynchronously creates a Paintable that is updated with the contents of the screen.
This function registers a new `socketpair()` which is shared with the QEMU instance to receive rendering updates. Those updates are
propagated to the resulting GdkPainable which can be retrieved using attach.end from
callback.
| this |
an Screen |
| cancellable | |
| callback |
a TaskReadyCallback to execute upon completion |
| user_data |
closure data for |