Some empty slot icons render translucent pixels as opaque

The bug

If textures used for empty slots contain translucent pixels, it will render these pixels as entirely opaque in-game in some cases. The results are inconsistent, with even the same texture file looking different in different situations.

This was originally mentioned in MC-109447, a ticket that has since been resolved as Won't Fix. However, MC-269085 was also part of that issue but was reported separately, and has since been fixed by Mojang, so I'm reporting this specific case separately as well (tickets combining a bunch of things together usually aren't good practice these days).

How to reproduce

1. Download and apply the attached resource pack
2. Look in the player's inventory, a loom, a smithing table, an enchanting table, or a tamed llama or horse inventory

Expected results

The slot icons would appear translucent, like their textures are.

Actual results

Sometimes they are, but it isn't consistent.

How to fix

Allow all of these cases to render translucent pixels.

Further notes

Since the slot icons are GUI textures and not items, wouldn't it make sense for the ones currently in the item textures directory to be relocated to the GUI sprites directory? Their current placement confuses me and is inconsistent.

There's other inconsistencies worth noting as well - see MC-265033.