filter-graph: add latency option to the convolver

Sometimes you want to use the convolver as a delay without adding
latency so add a latency option to tweak the automatic latency
reporting. Use this in the upmix rear delay filters.

src/daemon/filter-chain/sink-upmix-5.1-filter.conf

@@ -87,6 +87,7 @@ context.modules = [
 			    delay = 0.012
                             filename = "/hilbert"
 			    length = 33
+			    latency = 0.0
 			}
 		    }
                     {
@@ -101,6 +102,7 @@ context.modules = [
 			    delay = 0.012
                             filename = "/hilbert"
 			    length = 33
+			    latency = 0.0
 			}
 		    }
                  ]

src/modules/module-filter-chain.c

@@ -330,6 +330,7 @@ extern struct spa_handle_factory spa_filter_graph_factory;
  *                 length = ...
  *                 channel = ...
  *                 resample_quality = ...
+ *                 latency = ...
  *             }
  *             ...
  *         }
@@ -361,6 +362,8 @@ extern struct spa_handle_factory spa_filter_graph_factory;
  * - `channel` The channel to use from the file as the IR.
  * - `resample_quality` The resample quality in case the IR does not match the graph
  *                      samplerate.
+ * - `latency`  The extra latency in seconds to report. When left unspecified (or < 0.0)
+ *              the convolver latency will be the length of the IR.
  *
  * ### Delay
  *