World list buttons render translucent pixels as opaque

More loosely relates to these issues: MC-144009 MC-145821 MC-164001 MC-165403

The bug

None of the button textures used for the world select menu screen will render translucent pixels as translucent if provided a texture containing them.

How to reproduce

1. Download and apply the attached resource pack
2. Enter the singleplayer menu

Expected results

Translucent triangles.

Actual results

Opaque triangles.