Caused by error in refactor commit 4d668e6825. Offsets are drawn
outside of strip boundary, but strip content range in drawing context
is clamped by handles, so nothing was drawn.
Also this feature was not working, when using solo feature. This was
caused by another refactor e39cc78313. `special_seq_update` global
variable was declared in both files after splitting them, but set only
in one of them. Use `ED_sequencer_special_preview_get()` instead of
accessing variable directly.
16e2698632