Resolve `BorderRect` ambiguity

[ickshonpe]

Objective

In UI and world coordinates, “top” and “bottom” mean opposite things, so these fields are ambiguous. Anyone using BorderRect must decide themselves how to apply the vertical insets.

Fixes #21913

For more in detail motivation, see the replies to issue #21913

Solution

Replace the directional BorderRect fields (left, right, top, and bottom) with min_inset and max_inset Vec2 fields.

Using min_inset and max_inset removes the need to interpret top or bottom relative to the coordinate system, so the same logic will work consistently in both UI and 2D.