wayland: Use correct bounds size when computing the source box
The buffer transform specifies a transform from the buffer coordinate space to the surface coordinate space.
The inverse buffer transform specifies a transform from the surface coordinate space to the buffer coordinate space.
OutputTransform::map(QRect, QSizeF) expects both arguments to be in the same coordinate space.
In case of SurfaceInterfacePrivate::computeSourceBox(), both should be scaled surface coordinates so bufferTransform.inverted() maps the source rect to the proper buffer coordinate space.