fix two 1-bit mismatches in sRGB transfer functions

The Skia and skcms 'a' and 'b' terms disagree in the low bit.
'd' was exactly the same, but I've rewritten Skia's to match anyway.

Guarded by SK_LEGACY_SRGB_TRANSFER_FUNCTION.  *grumble*

Bug: skia:8278

Change-Id: Ie799f155cbe9c6a1fbe9043c827b8f79d04e1d96
Reviewed-on: https://skia-review.googlesource.com/150130
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed