Fix #133295: Grease Pencil tint influence vertexgroup has no effect

The weights were only used when "Use Weight as Factor" was used (this is
wrong though, that button uses the weights directly, otherwise the
Factor has to be multiplied by the weights -- as done in all other
modifiers [and in 4.2])

Pull Request: https://projects.blender.org/blender/blender/pulls/133297

source/blender/modifiers/intern/MOD_grease_pencil_tint.cc

@@ -187,11 +187,11 @@ static void modify_stroke_color(Object &ob,
   };
 
   auto get_point_factor = [&](const int64_t point_i) {
+    const float weight = vgroup_weights[point_i];
     if (use_weight_as_factor) {
-      const float weight = vgroup_weights[point_i];
       return invert_vertex_group ? 1.0f - weight : weight;
     }
-    return tmd.factor;
+    return tmd.factor * weight;
   };
 
   const GreasePencilTintModifierMode tint_mode = GreasePencilTintModifierMode(tmd.tint_mode);