* src/dispextern.h (struct image): Add font details required for the CSS.
* src/image.c (free_image): Free the font family string.
(search_image_cache):
(uncache_image): Make image caching understand the font details.
(lookup_image): Handle the font details when generating the image and
looking up the cache.
(svg_css_length_to_pixels): Handle 'em' when we know the font size.
(svg_load_image): Generate the CSS and apply it to the SVG.
(enum svg_keyword_index):
(svg_format):
(syms_of_image): Add ':css' attribute.
* doc/lispref/display.texi (SVG Images): Add details of new svg image
attributes.