openglrenderer: retrieve frame in each worker thread rather than in the proxy thread

Since we now have decoders in each thread, we can parallelize by retrieving
frames in each thread as well.

app/render/backend/opengl/openglproxy.cpp

@@ -67,7 +67,7 @@ bool OpenGLProxy::Init()
   return true;
 }
 
-void OpenGLProxy::FrameToValue(DecoderPtr decoder, StreamPtr stream, const TimeRange &range, NodeValueTable* table)
+void OpenGLProxy::FrameToValue(FramePtr frame, StreamPtr stream, const TimeRange &range, NodeValueTable* table)
 {
   // Ensure stream is video or image type
   if (stream->type() != Stream::kVideo && stream->type() != Stream::kImage) {
@@ -105,13 +105,6 @@ void OpenGLProxy::FrameToValue(DecoderPtr decoder, StreamPtr stream, const TimeR
 
     ColorManager::OCIOMethod ocio_method = ColorManager::GetOCIOMethodForMode(video_params_.mode());
 
-    FramePtr frame = decoder->RetrieveVideo(range.in(), video_params_.divider());
-
-    if (!frame) {
-      // Nothing to be done
-      return;
-    }
-
     // OCIO's CPU conversion is more accurate, so for online we render on CPU but offline we render GPU
     if (ocio_method == ColorManager::kOCIOAccurate) {
       bool has_alpha = PixelFormat::FormatHasAlphaChannel(frame->format());