Use BufferViewer for viewing constant buffers

* Unifying these views means that constant buffers have all the same
  reformatting and it avoids having multiple paths for what is now effectively
  the same control (a buffer can either have fixed data, repeating data, or
  both)

qrenderdoc/Widgets/Extended/RDHeaderView.cpp

@@ -122,6 +122,9 @@ void RDHeaderView::cacheSections()
 
   QAbstractItemModel *m = this->model();
 
+  if(!m)
+    return;
+
   int oldCount = m_sections.count();
   m_sections.resize(m->columnCount());