wayland: Explicitly initialize surface size to 0
QSize() will construct an invalid size. An invalid size has width and height set to -1. In other words, QSize() != QSize(0, 0). This can create issues when computing the bounding rect of a surface that has invisible subsurfaces.
For example, if the subsurface rect is QRect(0,0 -1x-1), the top-left corner of the bounding rect will be affected.
In order to make computation of the bounding rect robust, initialize the surface size to 0 explicitly.