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/signals.hpp

@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+#pragma once
+
+#include <functional>
+#include <utility>
+
+#include <wayland-server-core.h>
+
+namespace wlr {
+
+class SignalSink {
+public:
+	SignalSink() = default;
+	SignalSink(const SignalSink&) = delete;
+	SignalSink& operator=(const SignalSink&) = delete;
+
+	void connect(struct wl_signal* sig, std::function<void(void*)> fn) {
+		if (m_listener && m_listener->attached)
+			return;
+		if (!m_listener)
+			m_listener = new Listener;
+		m_listener->attached = true;
+		m_listener->fn = std::move(fn);
+		m_listener->listener.notify = &SignalSink::trampoline;
+		wl_signal_add(sig, &m_listener->listener);
+	}
+
+	void disconnect() {
+		if (m_listener && m_listener->attached) {
+			wl_list_remove(&m_listener->listener.link);
+			m_listener->attached = false;
+		}
+	}
+
+	~SignalSink() {
+		disconnect();
+		delete m_listener;
+	}
+
+private:
+	struct Listener {
+		struct wl_listener listener{};
+		std::function<void(void*)> fn;
+		bool attached = false;
+	};
+
+	static void trampoline(struct wl_listener* l, void* data) {
+		auto* self = wl_container_of(l, static_cast<Listener*>(nullptr), listener);
+		if (self->fn)
+			self->fn(data);
+	}
+
+	Listener* m_listener = nullptr;
+};
+
+}