Fix glitch on MNG animation Loops when QMovie caching is enabled

libmng delivers the last animation frame with a 1ms delay, and delivers
an extra blank frame after that with the proper delay time, but this
confuses QMovie when QMovie::cacheMode() == QMovie::CacheAll.

Task-Number: QTBUG-28894
Change-Id: Ic508c886286d9ec6786482927f321c6d77b68619
Reviewed-by: aavit <eirik.aavitsland@digia.com>

src/plugins/imageformats/mng/qmnghandler.cpp

@@ -263,6 +263,7 @@ mng_bool QMngHandlerPrivate::processHeader(mng_uint32 iWidth, mng_uint32 iHeight
 bool QMngHandlerPrivate::getNextImage(QImage *result)
 {
     mng_retcode ret;
+    const bool savedHaveReadAll = haveReadAll;
     if (haveReadNone) {
         haveReadNone = false;
         ret = mng_readdisplay(hMNG);
@@ -271,6 +272,13 @@ bool QMngHandlerPrivate::getNextImage(QImage *result)
     }
     if ((MNG_NOERROR == ret) || (MNG_NEEDTIMERWAIT == ret)) {
         *result = image;
+
+        // QTBUG-28894 -- libmng produces an extra frame at the end
+        //                of the animation on the first loop only.
+        if (nextDelay == 1 && (!savedHaveReadAll && haveReadAll)) {
+            ret = mng_display_resume(hMNG);
+        }
+
         frameIndex = nextIndex++;
         if (haveReadAll && (frameCount == 0))
             frameCount = nextIndex;