add xdg decoration, scene/tree factories, geometry + seat focus

more wrapper soup. xdg decoration manager, scene/tree node factories so
callers stop touching raw wlr_scene_*_create, a box geometry helper, and
seat pointer/keyboard focus clearing. dragging myself through it.

include/wlr/box.hpp

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+#pragma once
+
+#include <wlr/util/box.h>
+
+#include "wlr/wlr.hpp"
+
+namespace wlr {
+
+class Geometry {
+public:
+	Geometry() = default;
+	Geometry(int x, int y, int w, int h) : m_box{x, y, w, h} {}
+	explicit Geometry(const struct wlr_box& b) : m_box(b) {}
+
+	struct wlr_box* raw() { return &m_box; }
+	const struct wlr_box* raw() const { return &m_box; }
+
+	int x() const { return m_box.x; }
+	int y() const { return m_box.y; }
+	int width() const { return m_box.width; }
+	int height() const { return m_box.height; }
+
+	void set(int x, int y, int w, int h) {
+		m_box.x = x; m_box.y = y; m_box.width = w; m_box.height = h;
+	}
+
+	bool contains(double px, double py) const {
+		return wlr_box_contains_point(&m_box, px, py);
+	}
+	bool intersects(const Geometry& o) const {
+		struct wlr_box dest;
+		return wlr_box_intersection(&dest, &m_box, &o.m_box);
+	}
+
+private:
+	struct wlr_box m_box{};
+};
+
+}