Setting the Bézier handle of the Alpha key of a slot to over 100% or under 0% causes flickering.

[misaki-eymard]

Description:
When the Alpha Separation option is checked in the Slot settings, setting the Alpha Key curve to Bézier and adjusting the handle angle to point beyond 100% or below 0% causes flickering:

Expected behavior:
As shown in the curve view, when the curve reaches 100%, that slot becomes 100% alpha, and when it reaches 0%, it becomes 0% alpha.

Steps to reproduce:

1. Open spineboy-pro.spine using Spine 4.3.39-beta.
2. Switch to Animate mode, select the head slot, and check the Separate option for Alpha in the properties panel at the bottom of the Tree View.
3. Add alpha 100% and 0% keys at any timing in the animation.
4. Set the keys to Bézier curves, and position the handle of the alpha 100% key upward so the part of the curve aligns perfectly with 100%.
5. When you play the animation, you'll notice flickering occurs precisely when the curve reaches 100%.

The versions of Spine in which this problem was found:
4.2.43 and 4.3.39-beta

This issue was reported in the following thread on the forum:
https://esotericsoftware.com/forum/d/29168-transparency-of-attachment-below-0-or-above-100