NVIDIA 610.62 cannot sample R8G8_SRGB, and lavapipe can sample neither
it nor R8_SRGB, so an 8_8 sRGB T# made an image the device does not
support: creating it crashes the NVIDIA driver and lavapipe loads zeros.
The device's format support is read once. An 8 or 8_8 sRGB texture the
device cannot sample is created and viewed as R8_UNORM or R8G8_UNORM,
and SpirvTarget::srgbDecodeFormats makes the materializer mark the
image, so image loads map every channel through a 256-entry table of
the sRGB EOTF rounded to nearest even at bf16 precision, which is what
RDNA2 (gfx1035) returns; both channels of 8_8 are sRGB, and DST_SEL 0
and 1 map to themselves. Samples and gathers of such an image throw,
and its bindless table entries are unmapped like converted formats.
APS5_SRGB_SHADER_DECODE=1 takes this path on every device. Shader disk
cache entries and keys carry the mark: cache format version 7.
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