Black box when falling in hole in Libbet and the Magic Floor

[pinobatch]

If bit 7 of a sprite's attributes ($FE03, $FE07, $FE0B, ..., $FE9F) is true, the sprite's pixels are supposed to be drawn behind any background pixels that are not color 0. The comparison with color 0 is supposed to happen before BGP is applied.

To reproduce:

1. Open Libbet and the Magic Floor v0.04
2. Press Select to start the demo
3. Wait for Libbet to roll to the dead end at the top right corner

Expect: Libbet smoothly falls into the hole

Actual: A black square appears on the white cell below Libbet, which would otherwise have been hidden behind the background.