[FEAT] Render Message Flow with no message

[csouchet]

Closes #389

The position of the edge will be fixed with #351 and/or #349

[tbouffard]

@csouchet on the current screenshot, it seems that there is a transparency, z-order or fill color issue with the message flow start marker. We see the attached start shape on top of the circle i.e. a line crosses the circle.
I have seen this with the conditional sequence flow marker when zooming, but this doesn't occur often.

Start shape	current	may be expected
Activity
Pool
Event

[csouchet]

@csouchet on the current screenshot, it seems that there is a transparency, z-order or fill color issue with the message flow start marker. We see the attached start shape on top of the circle i.e. a line crosses the circle.
I have seen this with the conditional sequence flow marker when zooming, but this doesn't occur often.

Let's talk directly. I'm not sure that I understand what you say 😉

[csouchet]

@csouchet on the current screenshot, it seems that there is a transparency, z-order or fill color issue with the message flow start marker. We see the attached start shape on top of the circle i.e. a line crosses the circle.
I have seen this with the conditional sequence flow marker when zooming, but this doesn't occur often.
Start shape current may be expected
Activity
Pool
Event

If you see the BPMN specification, there is no obligation to have a filled circle:

I searched in the mxGraph's configuration, but we can't filled the STYLE_STARTARROW with a different color than that used for the stroke.

If we really want it, we need to create our own marker, like the Dash marker.

[tbouffard]

LGTM 👍
As discussed with @csouchet about the circle start marker (see #394 (comment)), we keep as it and may change it later if needed.