Improve the 3D editor selection box appearance

[Calinou]

• Draw two boxes slightly offset from each other to give the illustion of a thicker outline.
  • With large meshes, the boxes will appear to be more offset from each other. It looks a little strange, but at the same time, it also gives you a visual cue as to *how large the node's AABB is, which isn't necessarily a bad thing.
• Decrease the offset compared to the 3D node's AABB to give a more accurate representation of its size.
• Make the box fully visible instead of only displaying the corners.
• Draw a x-ray version of the box that's more translucent, but visible through walls. This helps make the box more visible while still having a sense of depth.
• Use an orange color similar to the 2D editor.

Preview

Old version

I thought about adding a subtle fill, but it's fairly complex so I'd prefer leaving it to a future PR.

[groud]

Same comment as in the 2D counterpart here : #40118
I think blue is not a distinctive color enough, so even if it's more consistent with the editor look, it kind of hurt legibility.
I would avoid a sky color as a selection outline.

The other changes look good :)

[Calinou]

@groud If you'd like, I can try drawing a translucent black box that's about as large as the original so the selection outline can be seen on any background.

[stijn-h]

The right edge looks like it is receiving shadows, or it is translucent (but it isn't behind any face). I don't think the selection box should receive shadows.

[Calinou]

@hinlopen Both selection boxes (translucent and opaque) are unshaded. The opaque selection box isn't visible through solid surfaces, but the translucent one is.

[Calinou]

I've tried to display both a black and white box with slightly different offsets, but it's not really easier to see:

Depending on the viewing angle, the lines may end up aligning perfectly, making one of them invisible.

[Calinou]

I pushed a new version which should be easier to see:

[akien-mga]

Thanks!

[groud]

Yup, I think it's a lot better with this orange! Good job!