Take cursor surface offset into account

From the spec

    On surface.attach requests to the pointer surface, hotspot_x
    and hotspot_y are decremented by the x and y parameters
    passed to the request. Attach must be confirmed by
    wl_surface.commit as usual.

In practice, I don't think it matters that much as most toolkits use
wl_pointer.set_cursor to change the hotspot.

src/cursorsource.cpp

@@ -124,6 +124,7 @@ KWaylandServer::SurfaceInterface *SurfaceCursorSource::surface() const
 void SurfaceCursorSource::refresh()
 {
     m_size = m_surface->size();
+    m_hotspot -= m_surface->offset();
     Q_EMIT changed();
 }