Toasts render translucent pixels as opaque

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

The bug

If the backgrounds for toasts translucent texture, they will render as completely opaque.

How to reproduce

1. Download and apply the attached resource pack
2. Make a toast appear

Expected results

The backgrounds would be translucent.

Actual results

They are opaque.

How to fix

Permit translucency for these textures in this context.