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[Skia] Text rendered as much less heavy compared to other browsers
https://bugs.webkit.org/show_bug.cgi?id=309152

Reviewed by Carlos Garcia Campos.

This change makes all the text rendered by skia look right - i.e.
just like in other browsers - by incorrectly blending in linear color
space despite target being in non-linear color space.

Until this patch, the rendering was done mathematically correct i.e.
the blending was done with respect to target's color space. However,
historically, many fonts were designed for incorrect blending and hence
they look the most "correct" in such a circumstances.

As skia internally implements so called "gamma hack", this change
tweaks SkSurfaceProps to properly tune it so that text looks correct.
Despite chromium uses 0.2 for contrast and 1.2 for gamma, this change
chooses 0 for contrast and 1 for gamma, as the results are visually
indistinguishable and yet in some reftests, the differences between
anti-aliased pixels are smaller. So in other words, this change makes
text anti-aliasing more stable (uniform) when it comes to very small,
sub-pixel positioning differences.

No new tests.

Canonical link: https://commits.webkit.org/308692@main

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