made output param cache thread safe

Multiple threads could be accessing these values at any given time, they'll
need to be thread safe.

app/node/output.cpp

@@ -41,26 +41,44 @@ QVariant NodeOutput::get_realtime_value()
 
 bool NodeOutput::has_cached_value(const TimeRange &time)
 {
-  return cached_values_.contains(time);
+  cache_lock_.lock();
+
+  bool contains = cached_values_.contains(time);
+
+  cache_lock_.unlock();
+
+  return contains;
 }
 
 QVariant NodeOutput::get_cached_value(const TimeRange &time)
 {
-  return cached_values_.value(time);
+  cache_lock_.lock();
+
+  QVariant val = cached_values_.value(time);
+
+  cache_lock_.unlock();
+
+  return val;
 }
 
 void NodeOutput::cache_value(const TimeRange &time, const QVariant &value)
 {
+  cache_lock_.lock();
   cached_values_.insert(time, value);
+  cache_lock_.unlock();
 }
 
 void NodeOutput::drop_cached_values()
 {
+  cache_lock_.lock();
   cached_values_.clear();
+  cache_lock_.unlock();
 }
 
 void NodeOutput::drop_cached_values_overlapping(const TimeRange &time)
 {
+  cache_lock_.lock();
+
   QHash<TimeRange, QVariant>::iterator i = cached_values_.begin();
 
   while (i != cached_values_.end()) {
@@ -70,4 +88,6 @@ void NodeOutput::drop_cached_values_overlapping(const TimeRange &time)
       i++;
     }
   }
+
+  cache_lock_.unlock();
 }