decoder/API: add decoder_read_much()

src/decoder/DecoderAPI.cxx

@@ -42,6 +42,27 @@ decoder_read(DecoderClient *client,
 	}
 }
 
+size_t
+decoder_read_much(DecoderClient *client, InputStream &is,
+		  void *_buffer, size_t size) noexcept
+{
+	uint8_t *buffer = (uint8_t *)_buffer;
+
+	size_t total = 0;
+
+	while (size > 0 && !is.LockIsEOF()) {
+		size_t nbytes = decoder_read(client, is, buffer, size);
+		if (nbytes == 0)
+			return false;
+
+		total += nbytes;
+		buffer += nbytes;
+		size -= nbytes;
+	}
+
+	return total;
+}
+
 bool
 decoder_read_full(DecoderClient *client, InputStream &is,
 		  void *_buffer, size_t size) noexcept