Respect disable-direct-scanout in DMA-BUF feedback

[ewgdg]

debug { disable-direct-scanout } should disable scanout path consistently, not only final direct-scanout attempt.

When direct scanout is disabled, advertising scanout-preferred DMA-BUF tranches no longer provides the intended benefit, but can still make clients choose scanout-oriented allocations.

This also intentionally keeps the regular render feedback render-only, even when the output is on the primary render node. Previously, in that case, niri reused the scanout feedback as the render feedback:

let render = if surface_render_node == Some(primary_render_node) {
    scanout.clone()
} else {
    builder.build()?
};

That made the default/render path contain scanout tranches too on the primary render node. As a result, Smithay's select_dmabuf_feedback() could not meaningfully choose between render feedback and scanout feedback, because both choices advertised scanout-oriented allocations.

This makes scanout feedback less aggressive in general: composited surfaces keep render-oriented feedback, while Smithay can still advertise scanout feedback for surfaces that are actually zero-copy candidates or failed scanout due to buffer properties.