Add C++20 RAII wrapper around wlroots 0.20 + scenefx

Wraps wlroots' C ABI behind owning types (Display, Backend, Compositor,
Output, XdgShell/Toplevel, Seat/Cursor/Keyboard, Scene/Node/Tree) with
wl_signal -> std::function callbacks. Links the scenefx effect engine and
ships a wlr::fx animation helper. Includes a runnable tinywl-style compositor
and a scene/blur demo.

License: AGPL-3.0-only.

include/wlr/backend.hpp

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+#pragma once
+
+#include <functional>
+
+#include "wlr/display.hpp"
+#include "wlr/signals.hpp"
+#include "wlr/compositor.hpp"
+
+namespace wlr {
+
+class Backend {
+public:
+	explicit Backend(Display& display, bool headless = false) {
+		m_backend = wlr_backend_autocreate(display.event_loop(), nullptr);
+	}
+
+	~Backend() {
+		m_newOutput.disconnect();
+		m_newInput.disconnect();
+		if (m_backend)
+			wlr_backend_destroy(m_backend);
+	}
+
+	Backend(const Backend&) = delete;
+	Backend& operator=(const Backend&) = delete;
+
+	struct wlr_backend* raw() const { return m_backend; }
+	explicit operator bool() const { return m_backend != nullptr; }
+	bool start() { return wlr_backend_start(m_backend); }
+
+	// cb receives struct wlr_output*
+	void on_new_output(std::function<void(void*)> cb) {
+		m_newOutput.connect(&m_backend->events.new_output, std::move(cb));
+	}
+	// cb receives struct wlr_input_device*
+	void on_new_input(std::function<void(void*)> cb) {
+		m_newInput.connect(&m_backend->events.new_input, std::move(cb));
+	}
+
+private:
+	struct wlr_backend* m_backend;
+	SignalSink m_newOutput;
+	SignalSink m_newInput;
+};
+
+}