Fixes #11141, a frame numbering mismatch in the commflagger.

The mismatch is between the frame numbering for seeking and the frame
numbering for the video display.  Also, optimize the "seek to next
frame" case.

This only affects the "experimental" commflagger.

mythtv/programs/mythcommflag/CommDetector2.cpp

@@ -598,9 +598,11 @@ bool CommDetector2::go(void)
             struct timeval start, end, elapsedtv;
 
             (void)gettimeofday(&start, NULL);
-            VideoFrame *currentFrame = player->GetRawVideoFrame(nextFrame);
+            bool fetchNext = (nextFrame == currentFrameNumber + 1);
+            VideoFrame *currentFrame =
+                player->GetRawVideoFrame(fetchNext ? -1 : nextFrame);
             long long lastFrameNumber = currentFrameNumber;
-            currentFrameNumber = currentFrame->frameNumber;
+            currentFrameNumber = currentFrame->frameNumber + 1;
             (void)gettimeofday(&end, NULL);
             timersub(&end, &start, &elapsedtv);
             timeradd(&getframetime, &elapsedtv, &getframetime);