[bdbarnett]

Fixes #245.

A timer-driven app like testris keeps running after its script body ends, kept alive from multimer's exit hook. RokuDisplay and RokuDisplayWrapper registered close() with atexit on their first frame, so it ran before that loop and quit the Companion channel while the app was still drawing. If the first frame came from a timer instead, the hook was registered during exit and never ran, so the channel was never closed.

close() now goes to multimer's stop hooks when multimer is keeping the program alive, and to atexit otherwise. show() returns early while close() runs: a timer could run it inside close()'s blocking ECP request, relaunch the channel and leave _launched set, so the next frame server was never fetched.

Checked:

• tests/test_roku_close.py is new. Both of its behaviour tests fail on the old code. The re-entrancy one fails with bind, launch, quit, launch, bind, the order the issue describes.
• A script with timer-driven frames on Linux CPython shows five frames, then the quit and the server closing. Before the fix it never closed.
• On MicroPython unix, a -c run still closes at exit.
• testris on Windows CPython against the Roku Companion channel, three 40-second runs: fetched 332, 396 and 199 frames (8.3, 9.9 and 5.0 fps), with no quit during the run. Before the fix, 3 runs in 4 fetched nothing.
• tests/test_cast_backends.py still passes.