treat SkPMColor as sRGB in SkPM4f::FromPMColor()

We made the wrong call in SkPM4f::FromPMColor().  SkPM4f::FromPMColor()
is only used by the color correct drawing pipeline, not legacy.  That
means it makes a lot more sense to treat SkPMColors as premul sRGB than
premul linear.

You can see the effect very clearly in any code path using the fallback
SkShader::Context::shadeSpan4f().  We shade legacy 8888, then
"linearize" to float by calling SkPM4f::FromPMColor().  At head we're
not really linearizing, which means everything ends up too bright in the
end.  Things get double sRGB-encoded, etc.

It is expected that this CL will make many color correct images look
darker and a lot more like legacy mode.  It may be jarring... we've
gotten used to seeing this bug and thinking brighter == fixed.

The only GM that changes in actual legacy 8888 is gamut, which
explicitly creates non-legacy 8888 images... the diff there is expected.

Change-Id: I77ac6cfe8f7ffb15e90f4aad798dbe8f9d3aafbd
Reviewed-on: https://skia-review.googlesource.com/15227
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>

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