We don't have lossy currentcolor in the style system anymore, except for a
single property -moz-font-smoothing-background-color.
I could've converted it into a proper StyleColor and thread down all the
necessary information to the font metrics code.
But it doesn't really seem worth it given it's not exposed to the web, so I just
did the simplest thing, which is making currentcolor compute to transparent to
that specific property.
This patch also removes the stores_complex_colors_lossily code and related,
since now we always can cache computed colors.
Differential Revision: https://phabricator.services.mozilla.com/D26187
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extra : moz-landing-system : lando
993731e44b