Can't use only alpha for transparency

I am trying to use a font with alpha transparency and black pixels. The black pixels get culled and I can't seem to find a way to make it render them over the background.

I've also tried explicitly setting the transparent color for the font to a different one than black, but it changes nothing:
```
let mut context = BTermBuilder::new()
.with_title("bracket-lib test")
.with_simple_console(80, 50, "tiles.png")
.with_font_bg("tiles.png", 16, 16, RGB::named(MAGENTA))
.with_tile_dimensions(16, 16)
.with_fullscreen(true)
.with_vsync(true)
.with_advanced_input(true)
.with_automatic_console_resize(true)
.build()?;
```