LibCSS now takes the client media spec, rather than just the
media type we're selecting for.
Signed-off-by: Michael Drake <michael.drake@codethink.co.uk>
Since the nscss_compute_font_size callback now needs to convert
new units to absolute values, and some of these units require info
from the root element's style, there are knock-on changes to ensure
that the required info is available where its needed.