KTX2Exporter: Fix metadata, add example

[donmccurdy]

Fixes some mistakes in KTX2Exporter-generated metadata, and adds an example closely based on the EXRExporter example. Tested by exporter PMREM textures and data textures, with output validated by https://github.com/KhronosGroup/KTX-Software.

# print info
ktx info texture.ktx2

# validate
ktx validate texture.ktx2

# extract to OpenEXR
ktx extract texture.ktx2 texture.exr

The raw float16 or float32 files exported from a PMREM texture tend to be large. They can be losslessly compressed with ZSTD to reduce the size somewhat...

ktxsc --zcmp 19 texture.ktx2

... but smaller file sizes would require more specialized compression, like BC6H or ASTC. The Basis Universal HDR UASTC spec, from Binomial, is a promising newer option available with KTX2 as well.

[donmccurdy]

I believe the changes to KTX2Exporter here are correct and necessary, but I'm unsure about adding an example nearly identical to the current EXRExporter example. I do want to have an example, though... should I just use a different scene/texture?

[Mugen87]

should I just use a different scene/texture?

That sounds good!

[donmccurdy]

Ok, done!