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.

examples/scene_demo.cpp

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+#include <cstdio>
+#include <cstdint>
+
+#include <wlr/scene.hpp>
+#include <wlr/node.hpp>
+#include <wlr/scenefx/anim.hpp>
+#include <wlr/scenefx/effect.hpp>
+
+int main(int argc, char** argv) {
+	(void)argc; (void)argv;
+
+	wlr::Scene scene;
+	if (!scene)
+		return 1;
+
+	wlr::Tree root{wlr_scene_tree_create(&scene.raw()->tree)};
+
+	const float color[4] = {1.0f, 0.2f, 0.2f, 1.0f};
+	wlr::Rect r{wlr_scene_rect_create(root.raw(), 200, 120, color)};
+	r.set_position(40, 60);
+
+	wlr::fx::Effect fx;
+	fx.position_y().set_duration(500);
+	fx.position_y().set_easing(wlr::fx::Easing::OutCubic);
+	fx.opacity().set_duration(300);
+	fx.opacity().set_easing(wlr::fx::Easing::InOutQuad);
+
+	fx.position_y().set(240.0f);
+	fx.opacity().set(0.3f);
+
+	std::uint64_t t = 0;
+	while (fx.animating()) {
+		t += 16;
+		fx.on_frame(t);
+		fx.apply_to(r);
+	}
+
+	wlr::Box box = {10, 20, 300, 40};
+	wlr::Box back = wlr::Box::from_box(box.to_box());
+	std::printf("wlroots++ demo: x=%d y=%d box=%dx%d anim=%s version=%s\n",
+		r.position().first, r.position().second,
+		back.width, back.height,
+		fx.animating() ? "yes" : "no",
+		wlr::version().c_str());
+
+	return 0;
+}