mxGraph outline may not use sketch style for peformance concern

[tbouffard]

With the initial implementation, the outline also use the sketch mode (even if some code is in place to avoid it)
We probably should disable it by default and add an option to enable it.
Notice that the draw.io implementation (at least the one of August 2020) also use sketch in mxgraph outline.

Generally, the outline style is simpler that the regular graph to improve performances: for instance, it doesn't display labels.