Replace "Invert" import option with more useful "Normal Map Invert Y"

[Calinou]

master version of #48693.

This can be used to invert a normal map's direction.

The "Invert" import option is no longer useful in Godot 4.0 since it uses height maps instead of depth maps in StandardMaterial3D. If you still need to use a depth map, SpatialMaterial3D features several properties to invert the height:

This closes godotengine/godot-proposals#785. This partially addresses #18299 (needs further testing).

Preview

Left: inverted
Right: not inverted

test_normal_map_direction.zip (build Godot with this PR before opening the project)

[clayjohn]

Wouldn't it be more consistent to move this option to SpatialMaterial3D? Seeing as we have the other flip options there already.

We need to keep in mind that Godot users upgrading from 3.2 to 4.0 may have designed their assets with Godot in mind.

[Xrayez]

This can be used to invert a normal map's direction.

If this change is welcome, I'll make a 3.2 version of this PR that keeps the "invert" import option for compatibility.

I'm not sure if it's safe to assume that this option is not used for something else, because as of now it's quite general-purpose. Even if it's doesn't make sense to have this in the engine as of now, I'm sure other modules/plugins could possibly benefit from the existing logic, as this could cover more than just ability to invert green channel, I'd personally keep invert option and add whatever is necessary for normal map processing.

[fire]

Is this pr considered rejected? @Xrayez makes an accurate point that normal fixing is different from invert.

[Calinou]

@fire I don't think we've gotten enough opinions about this yet. Also, this pull request is only for 4.0; I can make a PR for 3.x that keeps the current "Invert" import option while adding a "Invert green channel" option.

[fire]

I don't think more opinions will arrive. What is our consensus?

[akien-mga]

For the reference, we discussed this in a PR review meeting today and agreed with the change. We agreed that referencing Y instead of green would likely be better, though there was some concern about potential confusing with flipping the texture vertically on the Y axis. We suggested using normal_map_invert_y or similar to make it clear that it's in the context of normal maps where inverting Y is a common concept.

[akien-mga]

Thanks!