Improve connection focus visual

[hexawyz]

📝 Description of the Change

While toying around in the playground to understand how to customize connections, I noticed that the focus visual showed artifacts related to how the connection is drawn on screen.
I propose a small change to improve the focus visual so that it displays as a continuous outline.
Since only a single connection will be focused at a time, I believe the added computations are not going to slow down anything in a significant manner, but I might be wrong.

Before:

After:

🐛 Possible Drawbacks

This changes the appearance of the focus visual.
Although quite unlikely IMO, I guess it could have undesired results for some very specific custom connection implementations?

[hexawyz]

Sorry, I updated the PR with an additional improvement. 😢
The first version of the change was not enough, as it would still display interior details if the size of arrows was increased significantly from the default. (i.e. when the arrows are bigger than the pen used to generate the outline)

First try:

Now:

[miroiu]

Thank you very much for the PR. It looks great!