Rework recent, ff/rew, prebuffering changes.

Recent commits that fixed mediacodec ff/rew contained a large thinko
that inadvertently disabled the stuttering correction when starting
live TV.  This change fixes that regression and other small fixes
dealing with prebuffering.

Note that starting live TV still has a smaller, stuttering issue that
is unrelated to this change.  That issue will hopefully be addressed
doon.

Fixes #481

mythtv/libs/libmythtv/mythplayer.cpp

@@ -725,14 +725,20 @@ static bool preBufferDebug = qEnvironmentVariableIsSet("PREBUFFERDEBUG");
 
 bool MythPlayer::PrebufferEnoughFrames(int min_buffers)
 {
-    if (!m_videoOutput || !m_videoOutput->ValidVideoFrames())
+    if (!m_videoOutput)
         return false;
 
-    if (!(min_buffers
-          ? (m_videoOutput->ValidVideoFrames() >= min_buffers)
-          : (GetEof() != kEofStateNone
-             || m_videoOutput->EnoughDecodedFrames()
-             || (m_ffrewSkip != 0 && m_ffrewSkip != 1))))
+    auto wait = false;
+    if (min_buffers)
+        wait = m_videoOutput->ValidVideoFrames() < min_buffers;
+    else if (GetEof() != kEofStateNone)
+        wait = false;
+    else if (abs(m_ffrewSkip) > 1)
+        wait = !m_videoOutput->ValidVideoFrames();
+    else
+        wait = !m_videoOutput->EnoughDecodedFrames();
+
+    if (wait)
     {
         SetBuffering(true);
 
@@ -742,12 +748,13 @@ bool MythPlayer::PrebufferEnoughFrames(int min_buffers)
         // This code inserts a brief pause and play when the potential
         // for the jerking is detected.
 
-        bool watchingTV = IsWatchingInprogress();
-        if ((m_liveTV || watchingTV) && !FlagIsSet(kMusicChoice))
+        if ((m_liveTV || IsWatchingInprogress())
+            && !FlagIsSet(kMusicChoice)
+            && m_ffrewSkip == 1)
         {
             uint64_t frameCount = GetCurrentFrameCount();
             uint64_t framesLeft = frameCount - m_framesPlayed;
-            auto margin = static_cast<uint64_t>(m_videoFrameRate * 3);
+            auto margin = static_cast<uint64_t>(m_videoFrameRate * 3.0);
             if (framesLeft < margin)
             {
                 if (m_avSync.ResetAVSyncForLiveTV(&m_audio))
@@ -826,6 +833,11 @@ bool MythPlayer::PrebufferEnoughFrames(int min_buffers)
         return false;
     }
 
+    // Make sure we have at least one frame available.  The EOF case
+    // can get here without one.
+    if (!m_videoOutput->ValidVideoFrames())
+        return false;
+
     if (!m_avSync.GetAVSyncAudioPause())
         m_audio.Pause(false);
     SetBuffering(false);