Invisible layers on top prevent scan-out underneath

[emersion]

Have a layer that can be scanned out, and an invisible layer on top (CRTC_* describe a non-empty rectangle, but either the layer is disabled via FB_ID = 0 either it's completely transparent via alpha = 0).

libliftoff will prevent the bottom layer from being scanned out because of the check in has_composited_layer_over.