vs23: calibrateVsync(): repeat until we get a consistent result

The calibration sometimes returned incorrect results, leading to off
frame timings or mistaking of an NTSC system for PAL.

This version keeps sampling the timing until two consecutive measurements
with a difference below 80000 cycles (0.5 ms at 160 MHz) are taken.

ttbasic/vs23s010.cpp

@@ -185,12 +185,22 @@ void SMALL VS23S010::setMode(uint8_t mode)
 
 void VS23S010::calibrateVsync()
 {
-  uint32_t now;
+  uint32_t now, now2, cycles;
   while (currentLine() != 100) {};
   now = ESP.getCycleCount();
   while (currentLine() == 100) {};
   while (currentLine() != 100) {};
-  m_cycles_per_frame = m_cycles_per_frame_calculated = ESP.getCycleCount() - now;
+  for (;;) {
+    now2 = ESP.getCycleCount();
+    cycles = now2 - now;
+    if (abs(m_cycles_per_frame - cycles) < 80000)
+      break;
+    m_cycles_per_frame = cycles;
+    now = now2;
+    while (currentLine() == 100) {};
+    while (currentLine() != 100) {};
+  }
+  m_cycles_per_frame = m_cycles_per_frame_calculated = cycles;
 }
 
 void ICACHE_RAM_ATTR VS23S010::vsyncHandler(void)