hwconv: do not use the output texture as a default resource for the pipeline
hwconv: do not use the output texture as a default resource for the pipeline

This was not causing any issue in practice as we are overriding all the
texture bindings before calling ngli_pipeline_draw().

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl