Introduce SurfaceInterface::mapToChild()

The new method provides a convenient way to map a point from the
parent's coordinate system to a child's coordinate system.

src/wayland/surface_interface.cpp

@@ -999,4 +999,26 @@ void SurfaceInterface::handleBufferRemoved(BufferInterface *buffer)
     }
 }
 
+QPointF SurfaceInterface::mapToChild(SurfaceInterface *child, const QPointF &point) const
+{
+    QPointF local = point;
+    SurfaceInterface *surface = child;
+
+    while (true) {
+        if (surface == this) {
+            return local;
+        }
+
+        SubSurfaceInterface *subsurface = surface->subSurface();
+        if (Q_UNLIKELY(!subsurface)) {
+            return QPointF();
+        }
+
+        local -= subsurface->position();
+        surface = subsurface->parentSurface();
+    }
+
+    return QPointF();
+}
+
 } // namespace KWaylandServer

src/wayland/surface_interface.h

@@ -158,6 +158,14 @@ public:
      */
     QMatrix4x4 surfaceToBufferMatrix() const;
 
+    /**
+     * Maps the specified @a point in this surface's coordinate system to the equivalent point
+     * within the @a child's coordinate system, and returns the mapped point.
+     *
+     * If this surface is not an ancestor of the @a child, a null point is returned.
+     */
+    QPointF mapToChild(SurfaceInterface *child, const QPointF &point) const;
+
     void frameRendered(quint32 msec);
     bool hasFrameCallbacks() const;