filter-chain: add max builtin plugin

add max which takes two inputs and output the max value of them. it can
e.g. be used to create the same sidechain input to two mono limiters.

spa/plugins/filter-graph/builtin_plugin.c

@@ -2081,6 +2081,49 @@ static const struct spa_fga_descriptor param_eq_desc = {
 	.cleanup = free,
 };
 
+/** max */
+static void max_run(void * Instance, unsigned long SampleCount)
+{
+       struct builtin *impl = Instance;
+       float *out = impl->port[0], *in1 = impl->port[1], *in2 = impl->port[2];
+
+       if (out == NULL)
+               return;
+
+       unsigned long n;
+       for (n = 0; n < SampleCount; n++)
+               out[n] = SPA_MAX(in1[n], in2[n]);
+}
+
+static struct spa_fga_port max_ports[] = {
+       { .index = 0,
+         .name = "Out",
+         .flags = SPA_FGA_PORT_OUTPUT | SPA_FGA_PORT_AUDIO,
+       },
+
+       { .index = 1,
+         .name = "In 1",
+         .flags = SPA_FGA_PORT_INPUT | SPA_FGA_PORT_AUDIO,
+       },
+       { .index = 2,
+         .name = "In 2",
+         .flags = SPA_FGA_PORT_INPUT | SPA_FGA_PORT_AUDIO,
+       }
+};
+
+static const struct spa_fga_descriptor max_desc = {
+       .name = "max",
+       .flags = SPA_FGA_DESCRIPTOR_SUPPORTS_NULL_DATA,
+
+       .n_port = SPA_N_ELEMENTS(max_ports),
+       .ports = max_ports,
+
+       .instantiate = builtin_instantiate,
+       .connect_port = builtin_connect_port,
+       .run = max_run,
+       .cleanup = builtin_cleanup,
+};
+
 static const struct spa_fga_descriptor * builtin_descriptor(unsigned long Index)
 {
 	switch(Index) {
@@ -2128,6 +2171,8 @@ static const struct spa_fga_descriptor * builtin_descriptor(unsigned long Index)
 		return &sine_desc;
 	case 21:
 		return &param_eq_desc;
+	case 22:
+		return &max_desc;
 	}
 	return NULL;
 }

src/modules/module-filter-chain.c

@@ -466,6 +466,12 @@ extern struct spa_handle_factory spa_filter_graph_factory;
  * "Freq", "Ampl", "Offset" and "Phase" can be used to control the sine wave
  * frequency, amplitude, offset and phase.
  *
+ * ### Max
+ *
+ * Use the `max` plugin if you need to select the max value of two channels.
+ *
+ * It has two input ports "In 1" and "In 2" and one output port "Out".
+ *
  * ## SOFA filter
  *
  * There is an optional builtin SOFA filter available.