render one frame per thread for increased parallelism

If the nodes are now stateless, there's nothing stopping the renderer from
rendering multiple frames at once. Earlier since the nodes held some of their
input/output data (and that data could change per frame), it was not possible
to render multiple frames at once without conflicts. Now that the node state is
held in render threads, they can do whatever they want at any time.

app/render/backend/renderworker.cpp

@@ -4,17 +4,16 @@
 
 #include "node/block/block.h"
 
-RenderWorker::RenderWorker(DecoderCache *decoder_cache, QObject *parent) :
+RenderWorker::RenderWorker(QObject *parent) :
   QObject(parent),
   working_(0),
-  started_(false),
-  decoder_cache_(decoder_cache)
+  started_(false)
 {
 }
 
 bool RenderWorker::IsAvailable()
 {
-  return (working_ == 0);
+  return !working_;
 }
 
 bool RenderWorker::Init()
@@ -34,6 +33,8 @@ void RenderWorker::Close()
 {
   CloseInternal();
 
+  decoder_cache_.Clear();
+
   started_ = false;
 }
 
@@ -70,11 +71,6 @@ NodeValueTable RenderWorker::RenderAsSibling(NodeDependency dep)
   return value;
 }
 
-DecoderCache *RenderWorker::decoder_cache()
-{
-  return decoder_cache_;
-}
-
 NodeValueTable RenderWorker::RenderInternal(const NodeDependency &path)
 {
   return RenderAsSibling(path);
@@ -101,13 +97,13 @@ StreamPtr RenderWorker::ResolveStreamFromInput(NodeInput *input)
 DecoderPtr RenderWorker::ResolveDecoderFromInput(StreamPtr stream)
 {
   // Access a map of Node inputs and decoder instances and retrieve a frame!
-  DecoderPtr decoder = decoder_cache()->Get(stream.get());
+  DecoderPtr decoder = decoder_cache_.Get(stream.get());
 
   if (decoder == nullptr && stream != nullptr) {
     // Create a new Decoder here
-    DecoderPtr decoder = Decoder::CreateFromID(stream->footage()->decoder());
+    decoder = Decoder::CreateFromID(stream->footage()->decoder());
     decoder->set_stream(stream);
-    decoder_cache()->Add(stream.get(), decoder);
+    decoder_cache_.Add(stream.get(), decoder);
   }
 
   return decoder;