Use the base URI of the CSS style sheet for the background-image property.

Note the other CSS properties that take url values must be processed like this, too.

(CSSBackgroundImageValueImp::setValue) : Set the absolute URL to the internal uri field
(CSSParser::CSSParser) : Take the base URL as the default argument
(CSSParser::getURL) : New
(CSSParser::parse) : Set href of the style sheet to the specified base URL
(CSSParserTerm::getURL) : New
(CSSImportRuleImp::notify, HTMLLinkElementImp::linkStyleSheet, HTMLStyleElementImp::handleMutation) : Specify the base URL to the CSS parser
src/css/CSSGrammar.yy (term) : Set term.parser to the current CSS parser
testdata/css-008.html, testdata/css/css-008.css : New
testdata/css-009.html, testdata/css/css-009.css : New