Previously, ReadPixels, PBO upload/download, and clears would call cso_save_state with CSO_PAUSE_QUERIES, causing cso_context to call pipe->set_active_query_state() twice for each operation. This can potentially cause driver work to enable/disable statistics counters.
But often, there are no queries happening which need to be paused. By keeping a simple tally of active queries, we can skip this work.