Metal does not provide direct support for BC1_RGB formats
(VK_FORMAT_BC1_RGB_UNORM_BLOCK & VK_FORMAT_BC1_RGB_SRGB_BLOCK).
We have been faking it by mapping these Vulkan formats to Metal
formats containing alpha (MTLPixelFormatBC7_RGBAUnorm &
MTLPixelFormatBC7_RGBAUnorm_sRGB, respectively),
and advertising support for BC1_RGB formats.
However, this triggers CTS failures, because the BC1_RGBA formats can return
an alpha value of 0.0 when constructed that way, whereas the BC1_RGB formats
always expect 1.0 (opaque) to be returned.
This change moves to indirect support for BC1_RGB formats. They will still
be covered by MTLPixelFormatBC7_RGBAUnorm & MTLPixelFormatBC7_RGBAUnorm_sRGB,
and will effectively work (except transparency), but are no longer advertised
through physical device format and image format queries.
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