This improves the rendering of cyclical strokes in Grease Pencil
by connection the start and end lines segments together.
To make this possible the Vertex Shader needs to know if the
curve is cyclical and the start and end of the current stroke.
This PR stores the cyclical in the sign of `point index`. All points
already know the start of the curve as `stroke_id`, so all we
need is the end of the curve. Grease Pencil already uses a
point at the start and end of the stroke as padding. So if the
first buffer point stored the index to the last buffer point, any
other point and just go to the first point then to the last.
Pull Request: https://projects.blender.org/blender/blender/pulls/143976
fbbd79aa50