Fix blurry borders on antialiased StyleBoxFlat

[davthedev]

This is a fix of the antialiasing logic of StyleBoxFlat. It is now possible to have smooth rounded corners while keeping the edges sharp on the pixel, and remove the bleeding on the edges.

The problem is subtle but noticeable. In the current version, when the antialiasing is enabled on a StyleBoxFlat, the rounded corners are smoothed as expected. But, the sides also bleed a bit and lose their sharpness.

As the style boxes are mostly used to render UI components, it looks better when the sides are sharp on the pixel.

Before	After

Reference rendering in a vector graphics software

Since the box is rendered using colored GL triangles, the already-implemented trick to make the edges smooth is to use extra polygons with a fade-to-transparent. I have adjusted the positioning of those polygons so that the middle of the gradient matches the "hard" border, on both the ring and the infill.

This is based on principles found here: https://blog.mapbox.com/drawing-antialiased-lines-with-opengl-8766f34192dc

As the rendering principle may not be obvious, here is a visual explanation of how it works and the changes I have done to improve the rendering:

• Bugsquad edit: This closes The new default UI theme has blurry edges #75707.

[davthedev]

Resolves #75707

[Calinou]

Looks good to me. I'm slightly concerned about the default value change for antialiasing size, but most people don't change this particular property so it's probably not an issue for 4.1.

[davthedev]

If the previous value of 0.625 is used, you will get a slightly sharper corner edge. No big deal.

[Calinou]

If the previous value of 0.625 is used, you will get a slightly sharper corner edge. No big deal.

The default value isn't serialized in scene/resource files, so it's fine to have it set to 0.625 currently. This is only a concern for people who have changed the value from the default to any other value.

[davthedev]

This one should also be possible to backport to 3.x

[YuriSizov]

Thanks! And congrats on your first merged Godot PR!

[YuriSizov]

Cherry-picked for 4.0.3.

[lostminds]

If this changes the anti-aliasing of stylebox radius borders, does this also fix the old bug with incorrect overlaps with antialiasing on radius downsizing #68514 ?

[davthedev]

It is still there, just checked. This issue would require a different fix.