Fix loading projects saved before FilterObject.is_multi_filter was added.

--- a/Flowblade/persistancecompat.py
+++ b/Flowblade/persistancecompat.py
@@ -72,6 +72,10 @@ def FIX_MISSING_CLIP_ATTRS(clip):
         clip.link_seq_data = None 
 
 def FIX_MISSING_FILTER_ATTRS(filter):
+    if not hasattr(filter, "is_multi_filter"):
+        # Flowblade 2.24 saves this attr, but older project files do not have it.
+        filter.is_multi_filter = type(filter).__name__ == "MultipartFilterObject"
+
     if not hasattr(filter.info, "filter_mask_filter"):
         filter.info.filter_mask_filter = None
     if not hasattr(filter.info, "extra_editors_args"):