Use a 2-pixel outline in Viewport and Viewport-related editor icons

[Calinou]

This is more visually consistent with other editor icons.

• See Add viewport override to 2D editor #74573 (review)
  and Improve the visual consistency of Godot editor icons godot-proposals#15077.

Preview

Before	After

[aXu-AP]

What does the icon for SubViewport/-Container try to depict? The original quite clearly shows a rectangle inside another rectangle, where rectangle is the symbol for a viewport. The new has a circle inside a rectangle, so what does the circle represent?

[Calinou]

The new has a circle inside a rectangle, so what does the circle represent?

It's a small rectangle, but it looks like a circle due to the rounded corners. Unfortunately, it's not possible to make it have a non-square aspect ratio due to the limited space.

[Zireael07]

Maybe unround the corners then? It really does look like a circle

[aXu-AP]

I think this highlights why coming up with a robust set of guidelines for icons are needed (as stated in godotengine/godot-proposals#15077). If it isn't possible to make as simple icon as a rectangle inside another, the constraints are a bit too much. In this case, having 14x14 px space with 2 pixel wide lines/negative space (resulting essentially 7x7 grid for the icon) is super restrictive.