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/scenefx/effect.hpp

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+#pragma once
+
+#include <cstdint>
+#include <type_traits>
+
+#include "wlr/scenefx/anim.hpp"
+#include "wlr/scene.hpp"
+#include "wlr/node.hpp"
+
+namespace wlr::fx {
+
+class Effect {
+public:
+	Animated<>& position_x() { return m_x; }
+	Animated<>& position_y() { return m_y; }
+	Animated<>& scale()      { return m_scale; }
+	Animated<>& opacity()    { return m_opacity; }
+
+	void on_frame(std::uint64_t time) {
+		m_x.on_frame(time);
+		m_y.on_frame(time);
+		m_scale.on_frame(time);
+		m_opacity.on_frame(time);
+	}
+
+	void apply_to(Node& node) {
+		node.set_position((int)m_x.value(), (int)m_y.value());
+		if (node.type() != WLR_SCENE_NODE_BUFFER)
+			return;
+		Buffer b = node.as<Buffer>();
+		b.set_opacity(m_opacity.value());
+		const float s = m_scale.value();
+		const int dw = b.raw()->dst_width;
+		const int dh = b.raw()->dst_height;
+		if (s != 1.0f && dw > 0 && dh > 0)
+			b.set_dest_size((int)(s * dw), (int)(s * dh));
+	}
+
+	bool animating() const {
+		return m_x.animating() || m_y.animating() ||
+		       m_scale.animating() || m_opacity.animating();
+	}
+
+private:
+	Animated<> m_x;
+	Animated<> m_y;
+	Animated<> m_scale{1.0f};
+	Animated<> m_opacity{1.0f};
+};
+
+}