Add I2S::getOverUnderflow() (#1497)

See #1491.  Thanks @LinusHeu

docs/i2s.rst

@@ -84,6 +84,11 @@ void flush()
 ~~~~~~~~~~~~
 Waits until all the I2S buffers have been output.
 
+void getOverUnderflow()
+~~~~~~~~~~~~~~~~~~~~~~~
+Returns a flag indicating if the I2S system ran our of data to send on output,
+or had to throw away data on input.
+
 size_t write(uint8_t/int8_t/int16_t/int32_t)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Writes a single sample of ``bitsPerSample`` to the buffer.  It is up to the

libraries/I2S/src/I2S.h

@@ -54,6 +54,15 @@ public:
     virtual size_t write(const uint8_t *buffer, size_t size) override;
     virtual int availableForWrite() override;
 
+    // From the AR
+    bool getOverUnderflow() {
+        if (!_running) {
+            return false;
+        } else {
+            return _arb->getOverUnderflow();
+        }
+    }
+
     // Try and make I2S::write() do what makes sense, namely write
     // one sample (L or R) at the I2S configured bit width
     virtual size_t write(uint8_t s) override {