Auto merge of #11748 - emilio:stylo-background-xxx, r=mbrubeck

stylo: Support the rest of the background-xxx P1 properties.

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We do the same that with background-{image, repeat}, we just use the first
element until servo can parse and use a list efficiently.

cc/r? @mbrubeck @bholley @heycam

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

@@ -184,7 +184,7 @@ ${helpers.predefined_type(
 
 ${helpers.single_keyword("background-repeat", "repeat repeat-x repeat-y no-repeat")}
 
-${helpers.single_keyword("background-attachment", "scroll fixed")}
+${helpers.single_keyword("background-attachment", "scroll fixed" + (" local" if product == "gecko" else ""))}
 
 ${helpers.single_keyword("background-clip", "border-box padding-box content-box")}