Add basic KTX2 encoding support

[yue4u]

related: #6

This PR adds basic KTX2 encoding support (other KTX2 specific params like ktx2_zstandard_level are not added yet).

NOTE: Created .ktx2 files sometime produce a CustomMetadata warning when checked with ktx2check due to a workaround for potential alignment issue. (can be controlled by BASISU_DISABLE_KTX2_ALIGNMENT_WORKAROUND?)

Issues in: example_encoded_image.ktx2
    WARNING: Custom metadata "" found.

https://github.com/BinomialLLC/basis_universal/blob/ad9386a4a1cf2a248f7bbd45f543a7448db15267/encoder/basisu_comp.cpp#L2044

checks in KTX-Software

• https://github.com/KhronosGroup/KTX-Software/blob/fd4f0d0a1954816aef064fa1b4c03a6ec758c85a/tools/ktx/validate.cpp#L1460
• https://github.com/KhronosGroup/KTX-Software/blob/fd4f0d0a1954816aef064fa1b4c03a6ec758c85a/tools/ktx/validation_messages.h#L560

[dbdr]

Thanks a lot for your work on this!
I noticed basis-universal-sys/build.rs in your version still contains
.define("BASISD_SUPPORT_KTX2_ZSTD", "0")
So I suppose the generated ktx2 files won't use zstd compression? (I still need to try it out)