HDR ASTC texture feature "texture-compression-astc-hdr"

[mwyrzykowski]

We would like to propose an extension to the texture formats table and features for HDR ASTC textures.

Specifically under Section 25. Feature Index: https://gpuweb.github.io/gpuweb/#feature-index

"texture-compression-astc-hdr"

would be added as 25.11 after "bgra8unorm-storage"

When "texture-compression-astc-hdr" is supported, the following entries in https://gpuweb.github.io/gpuweb/#enumdef-gputextureformat would be supported:

    // ASTC HDR compressed formats usable if "texture-compression-astc-hdr" is both
    // supported by the device/user agent and enabled in requestDevice.
    "astc-4x4-hdr",
    "astc-5x4-hdr",
    "astc-5x5-hdr",
    "astc-6x5-hdr",
    "astc-6x6-hdr",
    "astc-8x5-hdr",
    "astc-8x6-hdr",
    "astc-8x8-hdr",
    "astc-10x5-hdr",
    "astc-10x6-hdr",
    "astc-10x8-hdr",
    "astc-10x10-hdr",
    "astc-12x10-hdr",
    "astc-12x12-hdr"

This feature is implementable in Metal (MTLPixelFormatASTC_[NxM]_HDR) and Vulkan VkPhysicalDeviceTextureCompressionASTCHDRFeatures. My understanding is D3D12 does not have support for ASTC HDR textures.