fix flatten stack splits contiguous clips issue and add tests

Signed-off-by: Ramesh Maddhu <[email protected]>
Signed-off-by: Mark Reid <[email protected]>

Author: rameshm

src/opentimelineio/stackAlgorithm.cpp

@@ -74,8 +74,7 @@ _flatten_next_item(
     }
 
     auto children = track->children();
-    int children_size = static_cast<int>(children.size());
-    for (int i = 0; i < children_size; i++)
+    for (size_t i = 0; i < children.size(); i++)
     {
         std::optional<TimeRange> trim = std::nullopt;
         SerializableObject::Retainer<Composable> child = children[i];
@@ -103,7 +102,7 @@ _flatten_next_item(
                 trim = child_trim;
             }
 
-            if (i+1 < children_size)
+            if (i+1 < children.size())
             {
                 child = children[i++];
             }