PMREMGenerator flips cube textures from WebGLRenderTargetCube

[jsantell]

Fixes #15301.

This is a straight forward solution but when checking different roughness values, it appears that the flipping is reversed in some mipmaps, but not sure where that's occurring in the code.

(the red circle should be on the left, yellow on the right, it's correct at roughness: 0 and several other times it appears)

As a quick test, flipping the X in the PMREMCubeUVPacker's frag color worked across all roughness values, but this seems that the fix is better off in the PMREMGenerator.

Thoughts?

[mrdoob]

/ping @spidersharma03 @bhouston @WestLangley

[bhouston]

I think the cubemap is flipped in threejs so we flipped the pmrem map as well. Check that threejs is consistent with itself. I believe it is or was the last time I checked. Thus it is just convention.

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[jsantell]

@bhouston my understanding is CubeTextures require a flip and WebGLRenderTargetCubes do not -- the PMREMGenerator assumes CubeTextures, so the flip is built in somewhere, and the places I've tried to handle this flip in this PR resulted in inconsistent flipping through the convolution layers (gif in OP) or only handled in the UV packer (which will work most of the time, but it should also have correct flip in the generator)

[jsantell]

Found the issue! The flip should only occur on the first LOD pass (which is currently handled, but only for CubeTexture). Changed queryScale to tFlip to be in line with other instances in three where a tFlip occurs, and now appropriately handle when receiving a texture from a WebGLRenderTargetCube. Updated PR -- this can be tested on the webgl_materials_envmap_exr.html (easier if changed to a SphereBufferGeometry) example just landed in #15299 (and webgl_materials_envmap_hdr.html to confirm CubeTexture PMREMs still work)

[mrdoob]

Thanks!