ComputedNode box model helper functions

[ickshonpe]

Objective

Add helper functions to ComputedNode that return the bounds of the node's border, padding, and content areas.

Solution

Add helper functions border_box, padding_box, and content_box to ComputedNode.

Included a release note. These changes are very trivial and don't really need one, but I've got quite a few more less trivial changes to ComputedNode that I want to add later.

Testing

This PR also includes a couple of trivial tests.

[kfc35]

This might be a stupid comment because I am new, but is this supposed to be out.min.y += self.border.bottom (and the next line should be out.max.y -= self.border.top)? Same comment about this belongs in content_box too

From reading Rect, I had the impression visually that Rect.min is the lower left point defined as (min.x, min.y) and Rect.max is the upper right point e.g (max.x, max.y), and that the BorderRect left/right/top/bottom values are units “towards its center.” If that’s the case, should min.y should be affected by border.bottom instead?

[ickshonpe]

It's confusing but, opposite to bevy_sprite, in bevy_ui the y-axis increases downwards. So Rect::min corresponds to the position of the top-left corner of the UI node and Rect::max corresponds to its bottom-right corner.

[kfc35]

🤦 Thanks for clarifying for me, I must commit this to memory!