shaders/colorspace: make gamut warning less broken

The current logic either just saturates the pixel or else does whatever.
I realized there's no really sane way to "invert" an out-of-gamut color,
and even if we did invert it in a reasonable manner, it's still less
visible than just making it pink.

Context provides better clues for what's out of gamut, anyways.
12 jobs for master in 7 minutes and 47 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #603125
docker aarch64

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passed linux #603121
docker amd64

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passed macos #603126
amd64 catalina

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passed scan #603127
docker amd64

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passed static #603122
docker amd64

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passed win32 #603123
docker amd64

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passed win64 #603124
docker amd64

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  Test
passed gpu #603129
gpu

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79.1142%
passed llvmpipe #603128
docker amd64

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  Sanitize
passed asan #603130
gpu

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passed msan #603131
gpu

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passed ubsan #603132
gpu

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