window-actor/wayland: Access single pixel buffer object directly

Using meta_wayland_single_pixel_buffer_from_buffer() can fail if the buffer resource is already gone. Given that we release the buffer immediately, that is actually expected - but the client this was tested with did/does not do so.

This made the single pixel black bar optimization fail for GstWaylandsink, while working for the mpv Wayland backend.

Access the object directly, which is both faster and works as long as the buffer is alive.

Fixes: ed50cbbf (window-actor/wayland: Update scanout candidate check)

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Closes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3381