Introduce noop backend

The noop backend is similar to headless, but it doesn't contain a
renderer. It can be used as a place to stash views for when there's no
physical outputs connected.

backend/noop/backend.c

@@ -0,0 +1,68 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <wlr/interfaces/wlr_output.h>
+#include <wlr/util/log.h>
+#include "backend/noop.h"
+#include "util/signal.h"
+
+struct wlr_noop_backend *noop_backend_from_backend(
+		struct wlr_backend *wlr_backend) {
+	assert(wlr_backend_is_noop(wlr_backend));
+	return (struct wlr_noop_backend *)wlr_backend;
+}
+
+static bool backend_start(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+	wlr_log(WLR_INFO, "Starting noop backend");
+
+	struct wlr_noop_output *output;
+	wl_list_for_each(output, &backend->outputs, link) {
+		wlr_output_update_enabled(&output->wlr_output, true);
+		wlr_signal_emit_safe(&backend->backend.events.new_output,
+			&output->wlr_output);
+	}
+
+	backend->started = true;
+	return true;
+}
+
+static void backend_destroy(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+	if (!wlr_backend) {
+		return;
+	}
+
+	struct wlr_noop_output *output, *output_tmp;
+	wl_list_for_each_safe(output, output_tmp, &backend->outputs, link) {
+		wlr_output_destroy(&output->wlr_output);
+	}
+
+	wlr_signal_emit_safe(&wlr_backend->events.destroy, backend);
+
+	free(backend);
+}
+
+static const struct wlr_backend_impl backend_impl = {
+	.start = backend_start,
+	.destroy = backend_destroy,
+};
+
+struct wlr_backend *wlr_noop_backend_create(struct wl_display *display) {
+	wlr_log(WLR_INFO, "Creating noop backend");
+
+	struct wlr_noop_backend *backend =
+		calloc(1, sizeof(struct wlr_noop_backend));
+	if (!backend) {
+		wlr_log(WLR_ERROR, "Failed to allocate wlr_noop_backend");
+		return NULL;
+	}
+	wlr_backend_init(&backend->backend, &backend_impl);
+	backend->display = display;
+	wl_list_init(&backend->outputs);
+
+	return &backend->backend;
+}
+
+bool wlr_backend_is_noop(struct wlr_backend *backend) {
+	return backend->impl == &backend_impl;
+}