new modes for grease pencil thickness modifier : curvature and intersecting

since in drawing you mostly draw thinner lines on convex curves and thicker lines on concave curves , a computed curvature could be used to thicken and thinnen grease pencil lines ..

obviously it also works for many of the other gp modifiers too, although its not a natural effect then but still interesting and useful.

another thing in drawing is that intersecting lines tend to draw a bit thicker where they touch. intersections could also be used to thicken lines around the intersection..