Add compat support for FFmpeg < 5.1

The new chlayout API has been introduced in FFmpeg 5.1. Use the old
channel_layout API on older versions.

app/src/util/average.c

@@ -0,0 +1,26 @@
+#include "average.h"
+
+#include <assert.h>
+
+void
+sc_average_init(struct sc_average *avg, unsigned range) {
+    avg->range = range;
+    avg->avg = 0;
+    avg->count = 0;
+}
+
+void
+sc_average_push(struct sc_average *avg, float value) {
+    if (avg->count < avg->range) {
+        ++avg->count;
+    }
+
+    assert(avg->count);
+    avg->avg = ((avg->count - 1) * avg->avg + value) / avg->count;
+}
+
+bool
+sc_average_get(struct sc_average *avg, float *value) {
+    *value = avg->avg;
+    return avg->count;
+}

app/src/util/average.h

@@ -0,0 +1,36 @@
+#ifndef SC_AVERAGE
+#define SC_AVERAGE
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+
+struct sc_average {
+    // Current average value
+    float avg;
+
+    // Target range, to update the average as follow:
+    //     avg = ((range - 1) * avg + new_value) / range
+    unsigned range;
+
+    // Number of values pushed when less than range (count <= range).
+    // The purpose is to handle the first (range - 1) values properly.
+    unsigned count;
+};
+
+void
+sc_average_init(struct sc_average *avg, unsigned range);
+
+/* Push a new value to update the "rolling" average */
+void
+sc_average_push(struct sc_average *avg, float value);
+
+/* Get the current average value (if available)
+ *
+ * An average is available if sc_average_push() has been called at least once.
+ */
+bool
+sc_average_get(struct sc_average *avg, float *value);
+
+#endif