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/output.c

@@ -0,0 +1,75 @@
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <wlr/interfaces/wlr_output.h>
+#include <wlr/util/log.h>
+#include "backend/noop.h"
+#include "util/signal.h"
+
+static struct wlr_noop_output *noop_output_from_output(
+		struct wlr_output *wlr_output) {
+	assert(wlr_output_is_noop(wlr_output));
+	return (struct wlr_noop_output *)wlr_output;
+}
+
+static void output_transform(struct wlr_output *wlr_output,
+		enum wl_output_transform transform) {
+	// empty
+}
+
+static bool output_make_current(struct wlr_output *wlr_output, int *buffer_age) {
+	return true;
+}
+
+static bool output_swap_buffers(struct wlr_output *wlr_output,
+		pixman_region32_t *damage) {
+	return true;
+}
+
+static void output_destroy(struct wlr_output *wlr_output) {
+	struct wlr_noop_output *output =
+		noop_output_from_output(wlr_output);
+
+	wl_list_remove(&output->link);
+
+	free(output);
+}
+
+static const struct wlr_output_impl output_impl = {
+	.transform = output_transform,
+	.destroy = output_destroy,
+	.make_current = output_make_current,
+	.swap_buffers = output_swap_buffers,
+};
+
+bool wlr_output_is_noop(struct wlr_output *wlr_output) {
+	return wlr_output->impl == &output_impl;
+}
+
+struct wlr_output *wlr_noop_add_output(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+
+	struct wlr_noop_output *output = calloc(1, sizeof(struct wlr_noop_output));
+	if (output == NULL) {
+		wlr_log(WLR_ERROR, "Failed to allocate wlr_noop_output");
+		return NULL;
+	}
+	output->backend = backend;
+	wlr_output_init(&output->wlr_output, &backend->backend, &output_impl,
+			backend->display);
+	struct wlr_output *wlr_output = &output->wlr_output;
+
+	strncpy(wlr_output->make, "noop", sizeof(wlr_output->make));
+	strncpy(wlr_output->model, "noop", sizeof(wlr_output->model));
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "NOOP-%d",
+		wl_list_length(&backend->outputs) + 1);
+
+	wl_list_insert(&backend->outputs, &output->link);
+
+	if (backend->started) {
+		wlr_output_update_enabled(wlr_output, true);
+		wlr_signal_emit_safe(&backend->backend.events.new_output, wlr_output);
+	}
+
+	return wlr_output;
+}