Use a size Vector for adjusting the size of Rectangles and Boxes

[madmiraal]

Currently, to adjust the size of a RectangleShape2D or a BoxShape3D you modify the extents, which are half the size of the rectangle or box. This is confusing for the user. Furthermore, although a BoxMesh3D (a CubeMesh in 3.2) uses a Vector3 for size, a CSGBox3D uses three floats: width, height and depth.

This PR allows a users to adjust the size of a RectangleShape, a BoxShape, a BoxMesh and a CSGBox consistently using a Vector property called size.

Part of #16863.

[aaronfranke]

It still says "Extents" in the corner when dragging the handles, I think this is not intentional:

For CSG, it says the name of which handle you are manipulating, but then the value is the size vector:

[madmiraal]

Updated to correctly display "Size" and the Vector3 in the corners.

[aaronfranke]

This code looks good, and I agree that having a size property makes sense. It's 100% an improvement for CSG.

However, there is an argument for having both size and extents properties. There are a lot of places in this code where size / 2 is used. More importantly, while a Rect2 and AABB have their position defined as the negative corner (top-left for Rect2, bottom-north-west for AABB), the position for BoxShape3D, RectangleShape2D, and CSG nodes is defined as the center, not the corner. To find the edges/corners of a Rect2 or AABB, you can take the position for one edge/corner, and the position plus size for the others. To do the same with BoxShape3D, RectangleShape2D, and CSG nodes, you would need to take the position and add/subtract the extents (half the size). So BoxShape3D currently behaves like Unity's Bounds.

Anyway, I would like @akien-mga's feedback on this (and anyone else who wants to chime in).

[akien-mga]

We discussed this in a PR review meeting and agreed with the change.

(Half) extents are still used internally for performance, but it makes sense to standardize everything in the higher level API around sizes.

[akien-mga]

Thanks!