common code: Fixed de-emphasis by adding high pass filter

This high-pass filter filters out DC and frequencies below 300 Hz.
Now de-emphases will not cause low frequencies to saturate audio level
range or cause too high levels on low frequencies.

src/common/emphasis.h

@@ -1,12 +1,14 @@
 typedef struct emphasis {
 	struct {
-		double last_value;
+		double x_last;
 		double factor;
 		double amp;
 	} p;
 	struct {
-		double last_value;
-		double factor;
+		double y_last;
+		double z_last;
+		double d_factor;
+		double h_factor;
 		double amp;
 	} d;
 } emphasis_t;