audioconvert: improve noise bits

Make a new noise method called PATTERN and use it to add a slow (every
1024 samples) repeating pattern of -1, 0.
Only use this method when we don't already use triangular dither.

See #2540

spa/plugins/audioconvert/fmt-ops.h

@@ -233,6 +233,7 @@ struct convert {
 #define NOISE_METHOD_RECTANGULAR	1
 #define NOISE_METHOD_TRIANGULAR		2
 #define NOISE_METHOD_TRIANGULAR_HF	3
+#define NOISE_METHOD_PATTERN		4
 	uint32_t noise_method;
 	float *noise;
 	uint32_t noise_size;