[2D] [Animation] Certain bones in a long chain are colored black when rigging

Steps to Repro:

- import a sprite and open the Sprite Editor Window
- go to the Bone Editor
- create a long chain of bones on the sprite (50+ bones)
- go to Skin Weights and Geometry Editor
- Generate geometry for the sprite
- Go to Weights and use Auto or Safe to quickly add weight

Notice that certain bones in the chain are colored black (bones 29, 35, 40, 41, 46, 47, 51, 52, 53, 57, 58, 59, 62, 63, 64, 65, 68, 69, 70, 71 will be black in a 100 bone chain).