scopes: only update if visible

Updating scopes can require heavy processing so we limit updates to only
when the scope is visible

app/widget/scope/histogram/histogram.cpp

@@ -49,7 +49,9 @@ void HistogramScope::SetBuffer(Frame* frame)
 {
   buffer_ = frame;
 
-  StartUpdate();
+  if (isVisible()) {
+    StartUpdate();
+  }
 }
 
 void HistogramScope::FinishedProcessing(QVector<double> red, QVector<double> green, QVector<double> blue)
@@ -111,6 +113,13 @@ void HistogramScope::StartUpdate()
   }
 }
 
+void HistogramScope::showEvent(QShowEvent* e)
+{
+  ManagedDisplayWidget::showEvent(e);
+
+  StartUpdate();
+}
+
 HistogramScopeWorker::HistogramScopeWorker() :
   cancelled_(false)
 {