GBA/OpenGL: Sprites behind a Mode 4 background are hidden entirely

[lunasorcery]

Any sprites with their priority set to draw behind a Mode 4 background will be hidden entirely - when populating the relevant background layer, the OpenGL renderer treats palette index 0 as a solid color rather than a transparent passthrough.
This might affect other background modes too; I'm not sure.

Observed on 7b3900f
Only occurs on the OpenGL Video Renderer. The Software Video Renderer is unaffected.

[endrift]

Do you have an example game this is used in? It's an easy fix but I want to make sure I do it properly.

[lunasorcery]

Observed on Asterix & Obelix XXL:

Software renderer for comparison:

and Drome Racers:

Software renderer for comparison: