Difficulty lock button renders translucent pixels as opaque

The bug

If the difficulty lock button is given a texture with translucent pixels, it will render these pixels as entirely opaque in-game, which is inconsistent with all other buttons, which support translucent pixels just fine.

How to reproduce

1. Download and apply the attached resource pack

2. Look at the options menu

Expected results

The difficulty lock button would not be a solid colour and would have a defined border and translucent center like all the other buttons do.

Actual results

The button is completely solid.

How to fix

Either allow the difficulty lock button to use translucent pixels, or separate the lock texture from the button texture and render them separately (this was what was done to fix MC-192307).