KTX2Loader: Fix mipmaps width and height.

[zajkowskie]

Fixed #25339

Description

Problem : Wrong mipmaps size (width and height) after KTX2Loader
Solution : Changed mipWidth and mipHeight to transcoded values.

[donmccurdy]

Something odd is happening on the webgl_loader_gltf_compressed example. It renders fine, but a bunch of warnings appear in the console:

GL_INVALID_VALUE: Offset overflows texture dimensions.

The only reason I can see is that origWidth/origHeight and width/height differ on the smallest mips:

mip	width x height	origWidth x origHeight
7	4 ⨉ 4	4 x 4
8	4 ⨉ 4	⚠️ 2 x 2
9	4 x 4	⚠️ 1 x 1

I'm not sure what WebGL expects. Maybe we need something more like this?

mipWidth = levelInfo.origWidth < 4 ? levelInfo.origWidth : levelInfo.width;
mipHeight = levelInfo.origHeight < 4 ? levelInfo.origHeight : levelInfo.height;

It should be safe to assume 4x4 blocks for current BasisU codecs.

[Mugen87]

Updated the PR with the code of #25375 (comment).

https://rawcdn.githack.com/zajkowskie/three.js/ebf6dfe327efc9d5bf1819e5e880ce2ed2971123/examples/webgl_loader_texture_ktx2.html
https://rawcdn.githack.com/zajkowskie/three.js/ebf6dfe327efc9d5bf1819e5e880ce2ed2971123/examples/webgl_loader_gltf_compressed.html

It seems the WebGL warning in webgl_loader_gltf_compressed is now gone.