Add a headless runner for iris examples

This VM has no display but does have a real GPU -- Vulkan 1.4 through
Venus and GL 4.6 through virgl, onto the host's card -- so the only thing
missing for a winit window is a compositor. Same trick `emu` uses for the
Android emulator: a headless sway, with grim for the picture.

It starts its own compositor rather than joining `emu`'s. sway tiles, so
adding a window to the one an emulator sits in resizes that emulator, and
a peer session's `emu up` could join at any moment. Xwayland is off here
because winit speaks Wayland; `emu` forces it on only because the Android
emulator's renderer speaks GLX.

It waits for the window to be mapped rather than sleeping a fixed time:
the first version's fixed sleep captured an all-black screen when sway
had started in the same invocation, which is indistinguishable from an
app that draws nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

iris/headless.conf

@@ -0,0 +1,14 @@
+# The compositor `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

iris/run-headless.sh

@@ -0,0 +1,108 @@
+#!/bin/sh
+# Run an iris example on this machine, which has no display.
+#
+#   ./run-headless.sh tabs [-- cargo args]
+#   ./run-headless.sh tabs --shot /tmp/tabs.png --seconds 4
+#
+# The VM has a virtio-gpu render node (Vulkan 1.4 through Venus, GL 4.6
+# through virgl), so wgpu runs on the host's real GPU -- what is missing is
+# only a compositor to give winit a surface. So: a headless sway, the same
+# trick `emu` uses for the Android emulator, and `grim` to see the result.
+#
+# It is deliberately *not* `emu`'s compositor. sway tiles, so adding a window
+# to the one an emulator is sitting in resizes that emulator's window, and a
+# peer session's `emu up` could join at any moment. This one has its own
+# socket and its own runtime directory and goes away with the machine.
+set -eu
+
+here=$(cd "$(dirname "$0")" && pwd)
+run="${XDG_RUNTIME_DIR:-/tmp}/iris-headless"
+seconds=3
+shot=""
+example=""
+
+while [ $# -gt 0 ]; do
+    case "$1" in
+        --shot) shot=$2; shift 2 ;;
+        --seconds) seconds=$2; shift 2 ;;
+        --) shift; break ;;
+        *) example=$1; shift ;;
+    esac
+done
+[ -n "$example" ] || { echo "usage: $0 EXAMPLE [--shot PNG] [--seconds N] [-- cargo args]" >&2; exit 2; }
+
+mkdir -p "$run"
+export SWAYSOCK="$run/sway.sock"
+
+# Named rather than left to sway's pid-based default, so a second run reuses
+# this compositor instead of starting another beside it.
+if ! swaymsg -t get_version >/dev/null 2>&1; then
+    rm -f "$SWAYSOCK"
+    WLR_BACKENDS=headless WLR_LIBINPUT_NO_DEVICES=1 LIBSEAT_BACKEND=noop \
+        setsid sway -c "$here/headless.conf" >"$run/sway.log" 2>&1 &
+    i=0
+    while [ $i -lt 20 ]; do
+        swaymsg -t get_version >/dev/null 2>&1 && break
+        i=$((i + 1)); sleep 0.5
+    done
+    swaymsg -t get_version >/dev/null 2>&1 || {
+        echo "run-headless: compositor did not start; see $run/sway.log" >&2
+        exit 1
+    }
+fi
+
+# Asked of the compositor rather than guessed: sway takes the first free
+# wayland-N, and this machine may already have one.
+rm -f "$run/display"
+swaymsg exec -- "sh -c 'printf %s \"\$WAYLAND_DISPLAY\" > $run/display'" >/dev/null
+i=0
+while [ $i -lt 20 ]; do
+    [ -s "$run/display" ] && break
+    i=$((i + 1)); sleep 0.5
+done
+[ -s "$run/display" ] || { echo "run-headless: could not read WAYLAND_DISPLAY" >&2; exit 1; }
+WAYLAND_DISPLAY=$(cat "$run/display")
+export WAYLAND_DISPLAY
+
+echo "run-headless: $WAYLAND_DISPLAY (sway $(swaymsg -t get_version --raw | sed -n 's/.*"human_readable":"\([^"]*\)".*/\1/p'))" >&2
+
+cd "$here"
+cargo build --example "$example" "$@" >&2
+bin="$here/target/debug/examples/$example"
+
+"$bin" >"$run/$example.log" 2>&1 &
+pid=$!
+trap 'kill "$pid" 2>/dev/null || true' EXIT INT TERM
+
+# Wait for the window to be mapped rather than for a number of seconds. A
+# fixed sleep took an all-black screenshot the first time this ran, when sway
+# had started in the same invocation and had not composited its output yet --
+# which is indistinguishable from an app that draws nothing.
+i=0
+while [ $i -lt 40 ]; do
+    kill -0 "$pid" 2>/dev/null || break
+    swaymsg -t get_tree --raw 2>/dev/null | grep -q "\"pid\":$pid," && break
+    i=$((i + 1)); sleep 0.25
+done
+
+# Then settle, for whatever the example does after its first frame.
+i=0
+while [ $i -lt "$((seconds * 2))" ]; do
+    kill -0 "$pid" 2>/dev/null || break
+    i=$((i + 1)); sleep 0.5
+done
+
+if kill -0 "$pid" 2>/dev/null; then
+    [ -n "$shot" ] && grim "$shot" && echo "run-headless: wrote $shot" >&2
+    kill "$pid" 2>/dev/null || true
+    wait "$pid" 2>/dev/null || true
+    status=0
+else
+    wait "$pid" 2>/dev/null || status=$?
+    echo "run-headless: $example exited early (status ${status:-0})" >&2
+    status=${status:-1}
+fi
+
+echo "--- $example output ---" >&2
+cat "$run/$example.log" >&2
+exit "$status"