hold worker busy state in renderbackend rather than in renderworker

Workers run in different threads and the backends can poll whether the worker
is currently busy or not. However the previous iteration has the worker (and an
atomic int) provide the busy state which could easily desync with the main
thread (since all workers run in different threads). By holding the busy states
in the main thread, the main thread will always be able to poll the busy state
accurately.

app/render/backend/renderworker.cpp

@@ -6,16 +6,10 @@
 
 RenderWorker::RenderWorker(QObject *parent) :
   QObject(parent),
-  working_(0),
   started_(false)
 {
 }
 
-bool RenderWorker::IsAvailable()
-{
-  return !working_;
-}
-
 bool RenderWorker::Init()
 {
   if (started_) {
@@ -51,9 +45,6 @@ NodeValueTable RenderWorker::RenderAsSibling(NodeDependency dep)
 
   //qDebug() << "Processing" << node->id();
 
-  // Set working state
-  working_++;
-
   // Firstly we check if this node is a "Block", if it is that means it's part of a linked list of mutually exclusive
   // nodes based on time and we might need to locate which Block to attach to
   if (node->IsTrack()) {
@@ -65,9 +56,6 @@ NodeValueTable RenderWorker::RenderAsSibling(NodeDependency dep)
 
   // We're done!
 
-  // End this working state
-  working_--;
-
   return value;
 }