Implement ASTC decoding with integrated decoder

[Erik-White]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

Adds support for decoding ASTC compressed KTX and KTX2 textures, including both LDR and HDR modes. Implementing an ASTC decoder was a major challenge, but I was able to base a lot of it on existing C++ implementations from ARM and Khronos.

ASTC is very complex, with different combinations of modes, block sizes and partitions, so I'm afraid that the diff is very large.

I have added tests that compare the output vs the ARM C++ implementation to ensure correctness (see ImageSharp.Textures.Astc.Reference.Tests), however this had to be a separate test project because the C# bindings wrapper (AstcEncoderCSharp) only supports .NET10.

Decoding speed compares favorably to the reference decoder, but I couldn't include those benchmarks here, again due to the .NET10 issue. You can find some comparison benchmarks in my separate implementation: https://github.com/Erik-White/AstcSharp

KTX1 has support for ASTC, but I couldn't find a tool to create test images and had a difficult time sourcing test data. So the tests are quite limited for v1, but much better for KTX2.

Limitations

• No support for 3D ASTC block types (4x4x4 etc)
• No encoding

Test data

Everything used is either created by myself, or sourced from
https://github.com/KhronosGroup/KTX-Software (Apache 2.0 license)
https://github.com/KhronosGroup/KTX-Software-CTS (Apache 2.0 license)
https://github.com/ARM-software/astc-encoder (Apache-2.0 license)

[Erik-White]

This keeps getting overwritten by a target that copies gitattributes from the shared project, so I will need to update it there as well

[Erik-White]

Casting to unit could result in an overflow when shifting. Widening to a long allows for correct shifting without overflow

[Erik-White]

AstcEncoderCSharp only supports .NET10 so I kept this in a separate test project. It is very useful to be able to compare output directly with the ARM implementation.

[Erik-White]

I wasn't quite sure whether to use uppercase or title case (HDR vs Hdr) for this namespace. Title case is mostly used elsewhere, but there are some excepts like IO