Implement sRGB support for ASTC decoding

[Erik-White]

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Description

Adds sRGB decoding support to the ASTC decoder, it was previously linear only. KTX/KTX2 *_SRGB_ASTC textures now decode with the ASTC spec sRGB endpoint expansion (RGB low byte 0x80, alpha stays linear) instead of linear replication.

There is a bit of churn in the existing test data where I have updated a couple of test methods to correct the (RGB/RGBA) naming. Some of the TestImage naming was also updated for better consistency

Background

KTX/KTX2 files carry ASTC data in two flavours, *_UNORM_BLOCK and *_SRGB_BLOCK. The block payload is identical but the format tag tells the decoder how to expand endpoints. Previously this decoder ignored that distinction and always used linear expansion, so *_SRGB_ASTC textures decoded with wrong pixel values.

Implementation

• LdrDecodeMode enum (Linear/Srgb) + ILdrColorMode strategy (LinearMode/SrgbMode) selecting per-channel 8 to 16-bit expansion
• Made the LDR decode path generic on (LdrPipeline, LdrPixelWriter, LogicalBlock.DecodeToBytes, FusedLdrBlockDecoder, SimdHelpers), AstcDecoder dispatches LinearMode vs SrgbMode
• Wired up KTX/KTX2 via 14 new RgbaAstcSrgb* block structs and repointed all *_SRGB_BLOCK switch arms to them