wayland: Fix SurfaceInterface::bufferSourceBox() when the viewport source rect is unset

Vscode starts with a valid source rect but it resets it eventually by
calling wp_viewport.set_source(-1, -1, -1, -1).

On the other hand, `current.viewport.sourceGeometryIsSet` would be true
if that happens.

SurfaceInterface::bufferSourceBox() should use the buffer size when the
source rect has been unset otherwise the damage handling code will be
confused and it's likely that the scene won't issue repaint requests.