Blender Git Loki

Grease Pencil sketches get included when doing OpenGL previews for scene strips in sequencer

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.