[vaapi] Always deinterlace when an interlaced frame was encountered once

PAFF video requires to run all frames through the deinterlacing filter
even if they are stored progressive.
After one interlaced frame was seen by the output, do not automatically
set the deinterlace method to NONE which would skip the filter.
Still, deinterlacing must be switched off if the user requests it.

Fixes xbmc#15817

xbmc/cores/VideoPlayer/DVDCodecs/Video/VAAPI.cpp

@@ -1862,6 +1862,7 @@ bool COutput::Init()
   m_config.processInfo->SetDeinterlacingMethodDefault(EINTERLACEMETHOD::VS_INTERLACEMETHOD_VAAPI_BOB);
 
   m_vaError = false;
+  m_seenInterlaced = false;
 
   return true;
 }
@@ -1982,9 +1983,13 @@ void COutput::InitCycle()
 
   EINTERLACEMETHOD method = m_config.processInfo->GetVideoSettings().m_InterlaceMethod;
   bool interlaced = m_currentPicture.DVDPic.iFlags & DVP_FLAG_INTERLACED;
+  // Remember whether any interlaced frames were encountered already.
+  // If this is the case, the deinterlace method will never automatically be switched to NONE again in
+  // order to not change deint methods every few frames in PAFF streams.
+  m_seenInterlaced = m_seenInterlaced || interlaced;
 
   if (!(flags & DVD_CODEC_CTRL_NO_POSTPROC) &&
-      interlaced &&
+      m_seenInterlaced &&
       method != VS_INTERLACEMETHOD_NONE)
   {
     if (!m_config.processInfo->Supports(method))

xbmc/cores/VideoPlayer/DVDCodecs/Video/VAAPI.h

@@ -275,6 +275,8 @@ class COutput : private CThread
   // extended state variables for state machine
   int m_extTimeout;
   bool m_vaError;
+  /// \brief Whether at least one interlaced frame was encountered in the video stream (indicating that more interlaced frames could potentially follow)
+  bool m_seenInterlaced;
   CVaapiConfig m_config;
   std::shared_ptr<CVaapiBufferPool> m_bufferPool;
   CVaapiDecodedPicture m_currentPicture;