IGNORE (Please avoid indexed textures using alpha, use RGBA)

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EDIT: Please ignore this.

With mip-mapping enabled, some GPUs (such as my Intel integrated graphics) cause a visual bug with indexed textures that use alpha. This bug has been present for a while but not noticed before. All indexed alpha textures in MTGame will now be converted to RGBA.
Issue https://github.com/minetest/minetest_game/issues/1397

We advise that in your mods you use RGBA mode for any texture that contains transparent or semi-tranparent pixels. You may wish to convert your existing textures.

[paramat]

With further testing, the visual bug remains even when textures are converted to RGBA, so there is no evidence doing this will help.