This new option applies the opacity using the strength of the stroke instead to use the alpha channel of the material.
Tested in greasepencil-object branch
{F7712796}
The vertex group filter has been removed because this filter is not logic in Material mode and must be valid only in Strength mode.
{F7713147}
Reviewers: pepeland, mendio
Reviewed By: mendio
Differential Revision: https://developer.blender.org/D5650