Fix unreported GPencil Thickness modifier affecting strokes not in Vertex Groups

Prior to this commit: If the Grease Pencil Thickness modifier is set to Normalize and a Vertex Group is selected, the thickness of all strokes are effected when changing the Thickness parameter. Points on strokes are only normalised (= pressure set to 1.0) if they are part of the Vertex Group; the strokes themselves may still change thickness.

With this patch: If Normalize is selected with a Vertex Group, Blender now pre-checks each stroke to determine whether it has vertices within or outside the Vertex Group. If all the points on the stroke belong to the Vertex Group, it normalises the whole stroke to a uniform thickness. If some or none of the points of the stroke belong to the Vertex Group, the stroke is now left as is.

Reviewed By: @antoniov
Differential Revision: https://developer.blender.org/D5483 with minor edit.