Don't request a visual playposition for an outdated frame

src/waveform/vsyncthread.cpp

@@ -38,6 +38,7 @@ void VSyncThread::run() {
     //qDebug() << "VSyncThread::run()";
     switch (m_vSyncMode) {
     case ST_FREE:
+        m_waitToSwapMicros = 1000;
         runFree();
         break;
     case ST_PLL:
@@ -65,8 +66,8 @@ void VSyncThread::runFree() {
         m_semaVsyncSlot.acquire();
 
         m_sinceLastSwap = m_timer.restart();
-        m_waitToSwapMicros = 1000;
-        usleep(1000);
+        m_waitToSwapMicros = m_waitToSwapMicros;
+        usleep(m_waitToSwapMicros);
     }
 }
 
@@ -187,6 +188,12 @@ void VSyncThread::setVSyncType(int type) {
 int VSyncThread::fromTimerToNextSyncMicros(const PerformanceTimer& timer) {
     int difference = static_cast<int>(m_timer.difference(timer).toIntegerMicros());
     // int math is fine here, because we do not expect times > 4.2 s
+    int toNextSync = difference + m_waitToSwapMicros;
+    while (toNextSync < 0) {
+        // this function is called during rendering. A negative value indicates
+        // an attempt to render an outdated frame. Render the next frame instead
+        toNextSync += m_syncIntervalTimeMicros
+    }
     return difference + m_waitToSwapMicros;
 }