Fixes:
[#18489] Adding (add filter) nested strips
causes Blender VSE to segfault on frame render.
[#18209] 3 VSE crash bugs with .blends
(all left click and move mouse over Metastrip/Blend Mode related)
... and a new segfault I introduced with my last commit.
Also: memcache limiter refcounts are tested now in critical
places, printing an error message,
since doing refcounting right is, well, hard.
960581fcc9