Fix ymfm timing issues by queuing register writes

Rapid register writes could cause audio glitches [1].
This introduces a queue to process one write per audio generation cycle.
This approach is the same as the one used in ymfm's vgmrender.cpp.

[1] https://github.com/aaronsgiles/ymfm/issues/1

src/fm/mako_ymfm.cpp

@@ -48,9 +48,8 @@ void MakoYmfm::Process(int16_t* stream, int len) {
 }
 
 void MakoYmfm::SetReg(RegType type, uint8_t addr, uint8_t val) {
-	uint32_t offset = type == OPNA_SLAVE ? 2 : 0;
-	opna.write(offset, addr);
-	opna.write(offset + 1, val);
+	uint8_t port = type == OPNA_SLAVE ? 2 : 0;
+	queue.push({port, addr, val});
 }
 
 void MakoYmfm::Generate(int16_t* buf, int samples) {
@@ -62,6 +61,12 @@ void MakoYmfm::Generate(int16_t* buf, int samples) {
 		// generate at the appropriate sample rate
 		assert(opna_pos <= output_pos);
 		while (opna_pos <= output_pos) {
+			if (!queue.empty()) {
+				auto w = queue.front();
+				opna.write(w.port, w.addr);
+				opna.write(w.port + 1, w.val);
+				queue.pop();
+			}
 			opna.generate(&opna_output);
 			opna_pos += opna_step;
 		}

src/fm/mako_ymfm.h

@@ -1,6 +1,7 @@
 #ifndef MAKOYMFM_H_
 #define MAKOYMFM_H_
 
+#include <queue>
 #include <vector>
 #include "ymfm_opn.h"
 #include "makofm.h"
@@ -15,6 +16,12 @@ private:
 	// we use an int64_t as emulated time, as a 32.32 fixed point value
 	using emulated_time = int64_t;
 
+	struct RegWrite {
+		uint8_t port;
+		uint8_t addr;
+		uint8_t val;
+	};
+
 	void SetReg(RegType type, uint8_t addr, uint8_t val) override;
 	void Generate(int16_t* buf, int samples);
 
@@ -24,6 +31,7 @@ private:
 	emulated_time output_pos = 0;
 	int last_sync = 0;
 	int samples_left = 0;
+	std::queue<RegWrite> queue;
 };
 
 #endif // MAKOYMFM_H_