Update RenderDoc API version to 1.2.0, add new function. Closes #1037

* The new function SetCaptureFileComments allows users to add comments to a
  capture after creating it at any time.
* We also use anonymous union to remove the need to duplicate API structs for
  backwards compatibility.

qrenderdoc/Code/CaptureContext.cpp

@@ -1026,6 +1026,7 @@ void CaptureContext::LoadBookmarks(const QString &data)
 
 bool CaptureContext::SaveNotes()
 {
+  // make sure this format matches SetCaptureFileComments in app_api.cpp if it changes
   QVariantMap root;
   for(const QString &key : m_Notes.keys())
     root[key] = m_Notes[key];