proxy: cleaned up, re-use the render cache's disk save system for proxies, multithreaded compression

app/codec/oiio/oiiodecoder.cpp

@@ -215,7 +215,7 @@ bool OIIODecoder::SupportsVideo()
   return true;
 }
 
-QString OIIODecoder::GetIndexFilename()
+QString OIIODecoder::GetIndexFilename() const
 {
   return QString();
 }

app/codec/oiio/oiiodecoder.h

@@ -45,7 +45,7 @@ public:
 
   virtual bool SupportsVideo() override;
 
-  virtual QString GetIndexFilename() override;
+  virtual QString GetIndexFilename() const override;
 
   static void FrameToBuffer(FramePtr frame, OIIO::ImageBuf* buf);