VSE: Fix movies are reloaded after invalidation

When cache is strip is invalidated, movie file was reloaded even if it
isn't necessary. This caused significant performance issues when strip
is being dragged under playhead.

This was caused by calling `SEQ_relations_sequence_free_anim()` and it
was introduced as fix for T36124.

When it is necessary to reload file because another API holds reference
to ImBuf, do this explicitly besides cache invalidation.

In `rna_ColorManagedColorspaceSettings_reload_update()` this was already
done, so no change is needed there.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11024

source/blender/sequencer/intern/strip_relations.c

@@ -114,7 +114,6 @@ static void sequence_invalidate_cache(Scene *scene,
   Editing *ed = scene->ed;
 
   if (invalidate_self) {
-    SEQ_relations_sequence_free_anim(seq);
     seq_cache_cleanup_sequence(scene, seq, seq, invalidate_types, false);
   }