enable-background is not parsed correctly

The tests/fixtures/reftests/bugs/587721-text-transform.svg test has an enable-background="new 0 0 1179.75118 687.74173" attribute in the toplevel svg element. This fails to parse, because our parser for that property expects new to not be followed by the dimensions of the background area. We need to support parsing them, at least. I don't know if the SVG spec specifies them as a hint only, or if they actually result in hard clipping for filters.

Relatedly, this is why parse_style_attrs() ignores errors when parsing presentation attributes: if we don't ignore them, that enable-background attribute causes that test to fail.