Bring the headless rig into the repository

A rendering claim about iris was verified by hand from another checkout,
because the compositor script and the input replay lived in ai-app's
submodule and not here.

`scripts/run-headless.sh` starts a headless sway on its own socket, runs
an example against it and screenshots the result. `rig-input`'s
`replay-touch` drives a recorded gesture in through Wayland's virtual
pointer, since a headless compositor has no input device to move.

`iris::harness` gains the `.touch` parser, so a recording means the same
thing replayed into a harness as into a window rather than being read
twice by two parsers.

The ai-app copy's `--phone` became `--mode`, since which phone is not
iris's business; its `IRIS_SCALE` has nothing to hand a density to here,
so it waits for one.

scripts/headless.conf

@@ -0,0 +1,14 @@
+# The compositor `scripts/run-headless.sh` starts, because this machine has no
+# display. Nothing here is meant to be looked at directly; `grim` is.
+#
+# No Xwayland: winit talks Wayland natively, and starting an X server is a
+# second thing to go wrong for no gain. (`emu`'s config forces it because the
+# Android emulator's renderer speaks GLX.)
+xwayland disable
+
+# A desktop-shaped output, since this is the desktop half of the port. Larger
+# than the window an example opens, so nothing is scaled or clipped.
+output HEADLESS-1 mode 1920x1200@60Hz
+
+default_border none
+focus_follows_mouse no