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/CMakeLists.txt

@@ -0,0 +1,5 @@
+add_executable(scene_demo scene_demo.cpp)
+target_link_libraries(scene_demo PRIVATE wlrootspp)
+
+add_executable(tinywl tinywl.cpp)
+target_link_libraries(tinywl PRIVATE wlrootspp)