rlmesh_variant_sink_push
RLMeshVariantSink is lent to enumerate_variants for that callback only; never stored or freed. Takes variant only on RLMESH_OK; on any other status the caller still owns it and frees it.
RLMeshStatus rlmesh_variant_sink_push(
RLMeshVariantSink* sink,
RLMeshVariant* variant
)