Gamemode switcher sprites render translucent pixels as opaque

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

The bug

selection.png and slot.png, used for the gamemode switcher, will render any translucent pixels in their textures as opaque. This is of particular note since the default textures for both of these use translucent pixels already, and are seemingly unintentionally rendered as solid in-game.

How to reproduce

1. Download and apply the attached resource pack

2. Use the gamemode switcher

Expected results

Translucent squares.

Actual results

Opaque squares.