void vrend_renderer_set_fence_retire(struct vrend_context *ctx,
                                     vrend_context_fence_retire retire,
                                     void *retire_data)
{
   assert(ctx->ctx_id);
   ctx->fence_retire = retire;
   ctx->fence_retire_data = retire_data;
}
void vrend_renderer_set_fence_retire(struct vrend_context *ctx,
                                     vrend_context_fence_retire retire,
                                     void *retire_data)
{
   assert(ctx->ctx_id);
   ctx->fence_retire = retire;
   ctx->fence_retire_data = retire_data;
}