Move samples of int16_t format to sample_t, that is of type double

This prepares the correction of all levels

src/common/sample.c

@@ -0,0 +1,27 @@
+
+#include <stdint.h>
+#include "sample.h" 
+
+void samples_to_int16(int16_t *spl, sample_t *samples, int length)
+{
+	while (length--) {
+		if (*samples > 32767.0)
+			*spl = 32767;
+		else if (*samples < -32767.0)
+			*spl = -32767;
+		else
+			*spl = (uint16_t)(*samples);
+		samples++;
+		spl++;
+	}
+}
+
+void int16_to_samples(sample_t *samples, int16_t *spl, int length)
+{
+	while (length--) {
+		*samples = (double)(*spl);
+		samples++;
+		spl++;
+	}
+}
+