[client] x11: added code to calibrate a delay for presentation

XPresent doesn't give us the time before presentation, but the time just
after. This code calculates and calibrates a delay to sleep for before
signaling the wait event for render when using jitRender

client/displayservers/X11/x11.c

@@ -1030,12 +1030,72 @@ static void x11XInputEvent(XGenericEventCookie *cookie)
 
 static void x11XPresentEvent(XGenericEventCookie *cookie)
 {
+#define CALIBRATION_COUNT 200
+  uint64_t deltats  = 0;
+  uint64_t deltamsc = 0;
+
   switch(cookie->evtype)
   {
     case PresentCompleteNotify:
     {
-      lgSignalEvent(x11.frameEvent);
       x11DoPresent();
+
+      XPresentCompleteNotifyEvent * e = cookie->data;
+
+      /* calibrate a delay to push our presentation time forward as far as
+       * possible without skipping frames */
+      static int      calibrate = 0;
+      static uint64_t lastts    = 0;
+      static uint64_t lastmsc   = 0;
+      static uint64_t delay     = 0;
+      if (lastts)
+      {
+        deltats  = e->ust - lastts;
+        deltamsc = e->msc - lastmsc;
+
+        if (calibrate == 0)
+        {
+          if (!delay)
+            delay = deltats / 2;
+          else
+          {
+            /* increase the delay until we see a skip */
+            if (deltamsc < 2)
+              delay += 100;
+            else
+            {
+              delay -= 100;
+              ++calibrate;
+            }
+          }
+        }
+        else
+        {
+          if (calibrate < CALIBRATION_COUNT)
+          {
+            /* every skip we back off the delay */
+            if (deltamsc > 1)
+            {
+              delay -= 100;
+              calibrate = 1;
+            }
+
+            /* if we have finished, print out the delay */
+            if (++calibrate == CALIBRATION_COUNT)
+              DEBUG_INFO("Calibration done, delay = %lu us", delay);
+          }
+        }
+      }
+
+      lastts  = e->ust;
+      lastmsc = e->msc;
+
+      /* minor adjustments if we are still seeing odd skips */
+      if (deltamsc > 1)
+        delay -= 10;
+
+      usleep(delay);
+      lgSignalEvent(x11.frameEvent);
       break;
     }
   }