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/videorenderframecache.cpp

@@ -46,7 +46,7 @@ void VideoRenderFrameCache::SetCacheID(const QString &id)
   cache_id_ = id;
 }
 
-QByteArray VideoRenderFrameCache::TimeToHash(const rational &time)
+QByteArray VideoRenderFrameCache::TimeToHash(const rational &time) const
 {
   return time_hash_map_.value(time);
 }
@@ -80,6 +80,21 @@ void VideoRenderFrameCache::Truncate(const rational &time)
   }
 }
 
+QList<rational> VideoRenderFrameCache::TimesWithHash(const QByteArray &hash)
+{
+  QList<rational> list;
+
+  QMap<rational, QByteArray>::const_iterator iterator;
+
+  for (iterator=time_hash_map_.begin();iterator!=time_hash_map_.end();iterator++) {
+    if (iterator.value() == hash) {
+      list.append(iterator.key());
+    }
+  }
+
+  return list;
+}
+
 void VideoRenderFrameCache::RemoveHashFromCurrentlyCaching(const QByteArray &hash)
 {
   currently_caching_lock_.lock();
@@ -87,7 +102,7 @@ void VideoRenderFrameCache::RemoveHashFromCurrentlyCaching(const QByteArray &has
   currently_caching_lock_.unlock();
 }
 
-QString VideoRenderFrameCache::CachePathName(const QByteArray &hash)
+QString VideoRenderFrameCache::CachePathName(const QByteArray &hash) const
 {
   QDir this_cache_dir = QDir(GetMediaCacheLocation()).filePath(cache_id_);
   this_cache_dir.mkpath(".");