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