decoder/renderer: no longer index automatically as part of the retrieve

functions

Indexing is a lengthy process and had a high chance of getting RenderWorkers
stuck doing it rather than being responsive to cache requests. This commit
introduces a system where workers never index media, but instead signal that
media is not ready to their RenderBackends which ensure that the media gets
indexed and re-queues the affected frames when those indexes are ready.

app/codec/oiio/oiiodecoder.cpp

@@ -133,7 +133,16 @@ bool OIIODecoder::Open()
   return true;
 }
 
-FramePtr OIIODecoder::RetrieveVideo(const rational &timecode, const QAtomicInt *cancelled)
+Decoder::RetrieveState OIIODecoder::GetRetrieveState(const rational &time)
+{
+  if (!open_ && !Open()) {
+    return kFailedToOpen;
+  }
+
+  return kReady;
+}
+
+FramePtr OIIODecoder::RetrieveVideo(const rational &timecode)
 {
   if (!open_ && !Open()) {
     return nullptr;
@@ -174,15 +183,6 @@ void OIIODecoder::Close()
   frame_ = nullptr;
 }
 
-int64_t OIIODecoder::GetTimestampFromTime(const rational &time, const QAtomicInt *cancelled)
-{
-  Q_UNUSED(time)
-
-  // A still image will always return the same frame
-
-  return 0;
-}
-
 bool OIIODecoder::SupportsVideo()
 {
   return true;