Textures for empty slots do not support translucent pixels

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

The bug

If textures used for empty slots are given translucent textures, they will incorrectly render as opaque in-game.

This concerns standalone sprites for empty slots - in most cases, the slots are baked into the UI texture (see MC-165182).

Affected slots

1. slot.png (used in Statistics and in superflat customization)

2. Bundle slots

3. Donkey/mule/llama chest slots

The saddle, armor and carpet slots used for horses, donkeys, mules, llamas and camels are also affected by this issue, but this specific case is covered under MC-269445. Making the slot item outline its own texture, and then baking the actual slot shape into the UI texture file (one for each mob), would fix MC-265033 and MC-270530.

How to reproduce

1. Download and apply the attached resource pack

2. Enter the statistics item menu, the superflat customization menu, a donkey/mule/llama's UI or mouse over a bundle

3. Observe

Expected results

Translucent slots.

Actual results

Entirely opaque.