Fix regression in commit 9d63be6.

The new filters were being added in front of all of the normalizing
filters, but they must come after. This was noticed because CPU video
filters were distorting the chroma planes with GPU processing enabled.
However, this problem affected all filters.

src/models/attachedfiltersmodel.cpp

@@ -266,7 +266,7 @@ void AttachedFiltersModel::add(QmlMetadata* meta)
 
         // Calculate the MLT index for the new filter.
         if (m_mltIndexMap.count() == 0) {
-            mltIndex = m_mltIndexMap.count();
+            mltIndex = m_producer->filter_count();
         } else if (insertIndex == 0) {
             mltIndex = m_mltIndexMap[0];
         } else {