After double checking the sequencer code, there doesn't seem to be any reason to
exclude these from the sequencer previews. This makes it possible to use the
sequencer to non-destructively chain together difference Grease Pencil animated
shots together without having to render each image sequence first, allowing for
a smoother workflow.
Just in case the initial assumption isn't entirely correct, I've put in place
an extra arg to the relevant functions which can be hooked up to a suitable
option on the scene strip later to turn this on/off as needed.
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