Blender Git Loki

Amendment to previous commit: Add an option to scene strips to disable GPencil

On second thought, it is probably still worthwhile to be able to disable GPencil
drawing on strips. By default, GPencil strokes are still shown by default now,
but they can be turned off using this option if it turns out that they are
getting in the way (e.g. a director/animator make some planning notes in the shot
at an earlier stage which are hidden for normal display now, but are still there
popping up sproadically during the animatic).