fix audio desync for realsies, stderr to log.txt

src/state.cpp

@@ -159,6 +159,31 @@ namespace anm2ed
   {
     auto currentTick = SDL_GetTicks();
     auto currentUpdate = SDL_GetTicks();
+    auto isRecording = manager.isRecording;
+    auto tickIntervalMs = (double)TICK_INTERVAL;
+
+    if (isRecording)
+    {
+      if (auto document = manager.get())
+      {
+        auto fps = std::max(document->anm2.info.fps, 1);
+        tickIntervalMs = std::max(1.0, 1000.0 / (double)fps);
+      }
+    }
+
+    if (isRecording != wasRecording)
+    {
+      // Drop any accumulated backlog when entering/leaving recording mode.
+      tickAccumulatorMs = 0.0;
+      previousTick = currentTick;
+      wasRecording = isRecording;
+    }
+
+    if (previousTick == 0) previousTick = currentTick;
+    auto tickDeltaMs = currentTick - previousTick;
+    tickDeltaMs = std::min<Uint64>(tickDeltaMs, 250);
+    tickAccumulatorMs += (double)tickDeltaMs;
+    previousTick = currentTick;
 
     if (currentUpdate - previousUpdate >= UPDATE_INTERVAL)
     {
@@ -167,10 +192,10 @@ namespace anm2ed
       previousUpdate = currentUpdate;
     }
 
-    if (currentTick - previousTick >= TICK_INTERVAL)
+    if (tickAccumulatorMs >= tickIntervalMs)
     {
       tick(settings);
-      previousTick = currentTick;
+      tickAccumulatorMs -= tickIntervalMs;
     }
 
     SDL_Delay(1);