Fix stalled renderAhead

If the worker can't render fast enough, the canvas doesn't
get updates because new frames are already outdated. So we
display the last available frame until it is replaced by a
new one.

src/subtitles-octopus.js

@@ -356,7 +356,8 @@ var SubtitlesOctopus = function (options) {
     }
 
     function _renderSubtitleEvent(event, currentTime) {
-        var eventOver = event.eventFinish <= currentTime;
+        // keep event displayed, if there is no gap after it, until it is replaced by a new one
+        var eventOver = event.eventFinish !== event.emptyFinish && event.eventFinish <= currentTime;
         if (self.oneshotState.eventStart == event.eventStart && self.oneshotState.eventOver == eventOver) return;
         self.oneshotState.eventStart = event.eventStart;
         self.oneshotState.eventOver = eventOver;
@@ -387,12 +388,17 @@ var SubtitlesOctopus = function (options) {
         if (!self.video) return;
 
         var currentTime = self.video.currentTime + self.timeOffset;
-        var finishTime = -1, eventShown = false, animated = false;
+
+        var eventToShow = null;
+        var finishTime = -1;
+        var animated = false;
+
         for (var i = 0, len = self.renderedItems.length; i < len; i++) {
             var item = self.renderedItems[i];
-            if (!eventShown && item.eventStart <= currentTime && (item.emptyFinish < 0 || item.emptyFinish > currentTime)) {
-                _renderSubtitleEvent(item, currentTime);
-                eventShown = true;
+
+            // we need the last started event
+            if (item.eventStart <= currentTime) {
+                eventToShow = item;
                 finishTime = item.emptyFinish;
             } else if (finishTime >= 0) {
                 // we've already found a known event, now find
@@ -404,16 +410,29 @@ var SubtitlesOctopus = function (options) {
                 } else {
                     break;
                 }
+            } else {
+                break;
             }
         }
 
-        if (!eventShown) {
-            if (Math.abs(self.oneshotState.requestNextTimestamp - currentTime) > EVENTTIME_ULP) {
-                _cleanPastRendered(currentTime);
-                tryRequestOneshot(currentTime, true);
+        var nextTime = currentTime;
+
+        if (eventToShow) {
+            _renderSubtitleEvent(eventToShow, currentTime);
+
+            if (finishTime >= 0) {
+                // request the next event from the most distant time
+                nextTime = Math.max(finishTime, nextTime);
+            } else {
+                // reached end-of-events
+                nextTime = -1;
             }
-        } else if (_cleanPastRendered(currentTime) && finishTime >= 0) {
-            tryRequestOneshot(finishTime, animated);
+        }
+
+        var freed = !self.video.paused && _cleanPastRendered(currentTime);
+
+        if ((freed || !eventToShow) && nextTime >= 0 && Math.abs(self.oneshotState.requestNextTimestamp - nextTime) > EVENTTIME_ULP) {
+            tryRequestOneshot(nextTime, nextTime === finishTime ? animated : true);
         }
     }