use less threads on cache

By using one thread per logical CPU thread, we seemed to completely saturate
the CPU which would kill the performance of the main/GUI thread (despite the
other threads being low priority). We now use half of the logical threads, which
still sees good CPU usage and minimal performance impact while allowing the
main thread to respond to user actions.

app/render/backend/renderbackend.cpp

@@ -20,7 +20,7 @@ bool RenderBackend::Init()
     return true;
   }
 
-  threads_.resize(QThread::idealThreadCount());
+  threads_.resize(qMax(1, QThread::idealThreadCount() / 2));
 
   for (int i=0;i<threads_.size();i++) {
     QThread* thread = new QThread(this);