Fix CollisionShape{2D,3D}.debug_color inconsistencies

[dalexeev]

• Fixes Default value of CollisionShape3D's property debug_color should not be saved in .tscn file #100088.
• See also Add debug_color property to CollisionShape2D #39072 (comment).

CC @BattyBovine

[akien-mga]

Wouldn't it be better to keep Color() (black) instead of transparent black as the fallback?

If using collision shapes without a scene tree, a black shape is probably better than a transparent one.

[dalexeev]

This is debatable. On the one hand, at least you'll see the shape. On the other hand, you might get a bunch of indistinguishable black rectangles, squares, spheres, etc. But in any case, this is a very hypothetical scenario, the vast majority of users use the standard SceneTree, and do not implement their own MainLoops.

[Zylann]

This transparent black already got noticed by a user of my voxel terrain module. They then though collision didn't work, but then we figured out it's because of the default value. Our system doesn't use nodes so we then had to add code to grab the debug_color from SceneTree.

[dalexeev]

Would you prefer transparent or black color?

[akien-mga]

Why can't we use a fallback value that's actually a good option, like the current default?

1837:   debug_collisions_color = GLOBAL_DEF("debug/shapes/collision/shape_color", Color(0.0, 0.6, 0.7, 0.42));

I think it's fine if hardcoded and not following user config as it's for more niche use cases, though from the report we got so far, more users seem to be affected by the transparent fallback that we thought.

Is this an issue this PR already addresses though, making all projects using SceneTree have the proper color (as opposed to only projects using shape nodes)?

[Zylann]

I would be for a better default too, although having skimmed over some code related to it, I think it's been used as a special value to mean "null". Maybe that should be checked as well

[akien-mga]

This is already better than the status quo, but maybe the fallback color could be something colored and semi-transparent like the previous default.

[akien-mga]

Let's merge as is for now and see if a different default is useful after dev7.

[akien-mga]

Thanks!