Fix grid snapping for box shape gizmos

[dervus]

Fixes #81916

I've rewritten Gizmo3DHelper logic for its box "extrusion" mode. In this version it applies snapping directly to box size and then adjusts box position by ensuring that opposite end of the box doesn't move as result of the operation. This way box size doesn't deviate from the snap grid.

Position (origin) adjustment logic is thus:

1. Calculate initial offsets of box sides on selected axis.
2. Adjust offset of the side we're extruding to accommodate new box size.
3. Adjust box position to be in the middle between unmoved side and extruded side.

[KoBeWi]

Shouldn't the gizmos get snapped to the grid?

godot.windows.editor.dev.x86_64_QZiiSPkNMR.mp4

They move in correct increments, but not sure if this is intended.

[dervus]

I'm quite new to Godot, so I'm not sure myself if it's intended or not, but AFAIK all translation ops with grid snapping just force target property to change in integer increments (0.1 increments if shifted). I think that makes sense for box sizing, though because when I'm working with several CSG shapes I find it handy if they all snap like this, so they can fit together even then their origin is not on grid.

[dervus]

On the other hand, I think it would be quite simple to make it always snap to visual grid. This would cause size property to deviate from being integer, but it always could be corrected by pulling the other side of the box. However, this would be inconsistent with translation gizmo behavior, and also would be a change from original behavior of this specific gizmo, so I assume even if it is desired, it shouldn't be a part of this PR?

[KoBeWi]

I assume even if it is desired, it shouldn't be a part of this PR?

Yeah, I guess.

[KoBeWi]

Seems fine.
CC @Calinou You might want to test the fixed behavior.

[akien-mga]

Could you squash the commits? See PR workflow for instructions.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[dervus]

Thanks for merging!

[JamesC01]

Thanks so much, just tested it and it seems to work well. This will make grey-boxing levels so much easier!