GroundedSkybox: Fix UVs being flipped

[hybridherbst]

Related issue: #27422, cc @elalish

Description

The current implementation had flipped UVs compared to using the same texture as environment.background and environment.image, which also resulted in flipped lighting.

I brought the GUI back and added a toggle to switch back and forth now :)

This contribution is funded by Needle

[hybridherbst]

Hm, may actually need to use the same logic as scene.background to decide if to flip:

boxMesh.material.uniforms.flipEnvMap.value = 
    ( background.isCubeTexture && background.isRenderTargetTexture === false ) ? - 1 : 1;

[Mugen87]

Regarding WebGLBackground: The flip depends on the texture type since textures of cube render targets have px/nx flipped.

[hybridherbst]

I see – then I think flipping here as currently in the PR is always correct for the supported case (an equirectangular texture is passed in), don't think passing in a cubemap would be expected to work for now.

[Mugen87]

https://rawcdn.githack.com/needle-tools/three.js/fix/grounded-skybox-uv-flip/examples/webgl_materials_envmaps_groundprojected.html

[elalish]

Ah, good catch! I totally missed that reversal in my model-viewer examples. I don't rely on cube map inputs, so this is fine, but it seems like adding that flip is still a good idea. Might as well make cube maps work too if it's easy.