KTX2Loader: Fix regression in mipmap chain upload

[donmccurdy]

• Fixes KTX2Loader: Offset overflows texture dimensions #25908
• Related KTX2Loader exmaple - Transcode wrong mipmaps size #25339
• Related KTX2Loader: Fix mipmaps width and height. #25375

Changes in #25375 allowed textures with non-multiple-of-four dimensions to display if the texture had no mipmaps, but caused a regression in mipmapped textures with non-power-of-two dimensions. As proposed in #25908, this PR aims to do both, but we cannot support non-multiple-of-four mipmapped textures in some texture formats and WebGL backends.

A warning will now be logged when loading a non-multiple-of-four texture with ETC1S or UASTC compression. Multiple-of-four dimensions are mandatory for KTX2 files used in glTF models, but the KTX2 format has no such restriction.

[donmccurdy]

Collection of test cases attached below:

KTX2 Variants.zip

To my understanding, the non_mul4_mips.ktx2 sample simply will not work on some devices with WebGL, without decoding to uncompressed RGBA8. We should strongly encourage users of KTX2 textures to choose multiple-of-four dimensions. Consider power-of-two dimensions, as well.