Add BCn encoders/decoders with RDO support

[walcht]

As discussed in #1159: ktxTexture2_CompressBCn and ktxTexture2_DecodeBCn are introduced in this PR to allow libktx users/consumers to encode/decode BCn textures from/to raw decompressed formats.

I still haven't added the RDO post processing step and I will once I finalize testing (which is currently just partial).
https://github.com/richgel999/bc7enc_rdo does not support BC6HU/BC6HS encoding/decoding and also no BC2 (this format is essentially dead since BC3 replaces it). Anyways, even though the names are ktxTexture2_CompressBCn and ktxTexture2_DecodeBCn only BC1, BC3, BC4, BC5, and BC7 are supported (I can add BC6HU/BC6HS support in another PR).

This is still a somewhat early PR. Please feel free to give feedback, edit, and nitpick as much as possible.

Some context: I am adding KTX2 support to OIIO (PR: AcademySoftwareFoundation/OpenImageIO#5185) and having libktx encode/decode BCn formats significantly simplifies things (also ETC encoding/decoding which I can also open a PR for - if approved).

I haven't updated the KTX-Software-CTS with the added BCn test files.

Once this is finalized, this will address issue #587.

Current TODOs:

• add CTS test files from a primary platform. As far as I understand, I shouldn't create/transcode KTX2 test files on non-primary platfroms (non arm64). I did do the testing on my local machine (see https://github.com/walcht/KTX-Software-CTS).
• add RDO post processing step
• add multithreading (only for encoder part; RDO already has multithreading integrated so here we are talking about the pre-processing step of encoding blocks).
• add BCn encoding support for ktx encode command
• add BCn decoding to PNG support for ktx extract command
• agree on which parameters to expose in ktxBCnParams struct (apparently there are many and I am not qualified to know which subset to expose or to expose them all). For the moment I will just expose them all.
• fix bc7enc_rdo compiler warnings (very simple to do, before merging git diff with original files so that we are 100% that we haven't changed anything).
• finalize test cases (not CLI tests but rather libktx tests - e.g., texturetests, etc.).
• enable SIMD acceleration for BC7 encoder (using ISPC - see https://github.com/ispc/ispc) (this should be straightforward and should be enabled by default via a CMake flag; e.g., BC7_SIMD).
• test LIBKTX_FEATURE_BCN_DECODER CMake flag option

Note1: no LLMs/AI coding tools were used in any capacity whatsoever in writing or aiding in the writing of this PR.
Note2: I am an individual contributor (main reason I am contributing here is to add support for KTX2 in Blender).

Edit: TODO list edits

[walcht]

There are also a lot of compiler warnings from bc7enc_rdo dependency. These should be straightforward to address directly in copied files from bc7enc_rdo.

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[MarkCallow]

@walcht, Thank you for this. Can you view the build logs?

One issue I notice immediately is that there are no changes to command_create.cpp. This too needs updating to support BCn encoding.

Regarding RDO and multi-threading, the UASTC encoder also has separate multi-threading options for the encoder and for the RDO step. You can follow the same model.

If we need to fix warnings in the encoder I suggest forking the encoder then incorporating the fork here by way of git subrepo. That will make it easier to potentially contribute fixes back upstream. If you are agreeable I can make the fork and provide instructions for how to incorporate it in your workarea.

Re. SIMD and ISPC, be careful how you support this. We need to support building and running on arm64 processors. Also compile flags to enable SIMD are not compatible with straightforward use of universal build tool chains, which is why this project does not do universal builds. Better is use of compiler pre-defined macros and run-time queries to discover what the software is being compiled for and running on. However since we aren't doing universal builds there is no need to obsess over this last detail.