Add support for ASTC compressed KTX textures

[Erik-White]

Prerequisites

• I have written a descriptive pull-request title
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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. A major challenge was finding a suitable ASTC decoder, I eventually decided to implement my own: https://github.com/Erik-White/AstcSharp
Most of the actual implementation is in the ASTC decoder, the changes in ImageSharp.Textures mostly just pass through to the decoder.

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

• Currently only supports LDR modes (low dynamic range)
• No support for 3D ASTC block types (4x4x4 etc)
• No encoding
• The ASTC decoder is currently fairly naïve and could do with performance improvements

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 is just to satisfy the tests that check the stream has been read to the end. The reader can jump around a bit in the file so it may not always end up positioned at the end otherwise.