The window_bounding_box of "linked-pipe-input" and "linked-pipe-output" is invalid (being super small but still positive). But since it is required for the entity I'd suggest removing definition of picture.window_background, picture.fluid_background, picture.flow_sprite and picture.gas_flow since they optional and not actually used because of such small window_bounding_box and therefor not even shown

I haven't paid attention to these things since they were first defined I tried it and found that it had no effect. Commenting out these codes should be a good idea.