[FEATURE REQUEST] Support `DXGI_FORMAT_BC7_UNORM_SRGB` DDS format

[MiroslavR]

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OIIO currently supports DXGI_FORMAT_BC7_UNORM but not DXGI_FORMAT_BC7_UNORM_SRGB. The DDS reader reports Unsupported compression type for the latter.

Here's an example DDS file in that format: example_BC7_UNORM_SRGB.dds.zip (GitHub forced me to zip it).