GPencil: Set Viewport color to black by default

As the 2D viewport color is white, if the object default color is white, when wireframe is enabled the object is invisible.

Now, the grease pencil object default viewport color is black.

source/blender/editors/object/object_add.c

@@ -1066,6 +1066,9 @@ static int object_gpencil_add_exec(bContext *C, wmOperator *op)
 
 	/* if this is a new object, initialise default stuff (colors, etc.) */
 	if (newob) {
+		/* set default viewport color to black */
+		copy_v3_fl(ob->color, 0.0f);
+
 		ED_gpencil_add_defaults(C, ob);
 	}