style: Remove the last usage of lossy currentcolor.

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

components/style/properties/longhands/background.mako.rs

@@ -9,7 +9,7 @@
 ${helpers.predefined_type(
     "background-color",
     "Color",
-    "computed_value::T::transparent()",
+    "computed::Color::transparent()",
     initial_specified_value="SpecifiedValue::transparent()",
     spec="https://drafts.csswg.org/css-backgrounds/#background-color",
     animation_value_type="AnimatedColor",