Better inset border SDF

## What problem does this solve or what need does it fill?

Currently `sd_inset_rounded_box` in [ui.wesl](https://github.com/bevyengine/bevy/blob/7b2b90997ae2e73558ace09cc065d31a2c24b266/crates/bevy_ui_render/src/ui.wesl) positions center of inset box at an offset to center of outside box. This can cause incorrect behavior in some edge cases:

Note: this screenshot was made with bevy 0.19

## What solution would you like?

Using sdf of an outset box it is possible to have two different radii meet:

The solution is implemented in shadertoy:
[Border using an "outset" inset box](https://www.shadertoy.com/view/Ncy3Dy)

## What alternative(s) have you considered?

Cannot think of other solution as of now

## Additional context

Problem exists since sdf of a rounded box is calculated in the same quadrant because `sd_rounded_box` takes an absolute position of a point. This does not allow different radii to meet at the center. If we treat each side as different distance from center, this does not happen.

Start in crates/bevy_ui_render/src/ui.wesl at sd_inset_rounded_box, then compare its distance calculation with the linked Shadertoy implementation. Verify the change handles different radii meeting at the center and produces the expected upper-right corner behavior shown in the screenshots.

Written by the indexing model from the issue text.