add attenuation example

gui/examples/8_attenuation.cpp

@@ -0,0 +1,12 @@
+Sample* process_data(Samples samples)
+{
+	const auto p = param1();
+	const float scale = p / 4095.f;
+	const auto offset = (4095 -	p) / 2;
+
+	for (int i = 0; i < SIZE; ++i)
+		samples[i] = samples[i] * scale + offset;
+
+    return samples;
+}
+