Only for BC5 and BC6H formats for now.
However, adding new formats is easy:
1. Copy format's definition from `VkFormat` to `ktx2.h`
2. Map it to `PF_` format in `Image_KTX2Format()`
3. Done
4. Really, that should be it.
5. I mean, most formats would require PF_ enum extension, but we're not
going to support all of them right?
It only does a very basic header validation, and passes the entire file
as PF_KTX2_RAW format. This is to simplify KTX2 reading in ref_vk and
trying out different formats. KTX2 has support for >200 format, and
passing all of them through PF_ types is a non-starter.
The plan is to figure out which formats we want to support, and add
their support to imagelib/ktx2 incrementally, leaving the rest as
PF_KTX2_RAW, so ref_vk still can use them.
Ivan Avdeev