Auto merge of #19105 - mstange:move-fsbgcolor, r=emilio`

Bug 1414147 - Move fontSmoothingBackgroundColor from nsStyleUserInterface to nsFont, stylo changes.

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This will need to land in concert with [bug 1414147](https://bugzilla.mozilla.org/show_bug.cgi?id=1414147). Please don't r+ until the patch in that bug has been r+ed.

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