GfxCal*ColorSpace: introduce Bradford transform for chromatic adaptation

This brings the lcms2 code path and the non-lcms2 code path closer to each
other in terms of color reproduction.
So far the following points were missing, which are now added in this commit:
- Both code paths did either not adjust for a different source white point at all (lcms2 code path)
  or did some simple scaling. The code has now been adjusted to use the Bradford transform
  to adapt to either the D50 or D65 white point, depending on the code path.
- The non-lcms2 code path so far used the square root as an approximate gamma function.
  The correct sRGB gamma function has now been added.
8 jobs for bradfordtransform in 10 minutes and 33 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Build
passed #6813758
build

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build_android

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build_clang_libcpp

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build_clazy

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build_mingw64_fedora

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build_ubuntu_16_04

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clang_format

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build

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