[MC-269445] 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

Download and apply the attached resource pack
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.

Slot	Result
Player armor slots (Survival)	❌ Opaque
Player offhand slot (Survival)	❌ Opaque
Player armor slots (Creative)	❌ Opaque
Player offhand slot (Creative)	❌ Opaque
Enchanting table lapis lazuli slot	❌ Opaque
Smithing table template slot	✔ Translucent
Smithing table material slot	✔ Translucent
Smithing table input item slot	✔ Translucent
Loom banner slot	❌ Opaque
Loom banner pattern slot	❌ Opaque
Loom dye slot	❌ Opaque
Brewing stand fuel slot	❌ Not individually texturable (MC-74408)
Brewing stand bottle slot	❌ Not individually texturable (MC-74408)
Horse/donkey/mule saddle slot	❌ Opaque
Horse armor slot	❌ Opaque
Llama carpet slot	❌ Opaque

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.