VSE: Reduce memory footprint when using thumbnails

Free strip `anim` data immediately after rendering. This doesn't affect
rendering performance, because each new loop would have to seek to start
of strip. Also strips are now freed anyway, but after rendering loop
ends.

With SF edit file, thumbnail rendering used around 60GB of memory.
Now it uses few hundreds MB (depends on movie file resolution, codec,
etc.)

Freeing of strips caused UI to be unresponsive for brief period. This
issue is not removed, but is more spread out so it is less noticable.

source/blender/editors/space_sequencer/sequencer_thumbnails.c

@@ -36,6 +36,7 @@
 
 #include "BIF_glutil.h"
 
+#include "SEQ_relations.h"
 #include "SEQ_render.h"
 #include "SEQ_sequencer.h"
 
@@ -176,6 +177,7 @@ static void thumbnail_start_job(void *data,
       SEQ_render_thumbnails(
           &tj->context, val->seq_dupli, seq_orig, start_frame, frame_step, tj->view_area, stop);
       SEQ_render_thumbnails_base_set(&tj->context, val->seq_dupli, seq_orig, tj->view_area, stop);
+      SEQ_relations_sequence_free_anim(val->seq_dupli);
     }
     BLI_ghashIterator_step(&gh_iter);
   }