Partially translucent fonts don't render properly on signs

The bug

If a resource pack specifies that a font that has characters whose texture is partially translucent, the translucent pixels will prevent the sign from rendering behind them.

To reproduce

1. Use this example resource pack: https://drive.google.com/file/d/1eT1inIgiRceem5oZWHSfhsskV4rBgkLf/view
2. Write something on a wall sign attached to some white blocks
   → You'll see a white border around all characters