Blender Git Loki

Bugfix T43293: Crash when editing shared GPencil datablock in VSE

The problem here was that when a Grease Pencil datablock is shared between
the 3D view and another one of the editors, all the strokes were getting handled
by the editing operators, even if those strokes could not be displayed/used
in that context. As a result, the coordinate conversion methods would fail,
as some of the needed data would not be set.

The fix here involves not including any offending strokes in such cases...