src/svgtiny_css.c: new file with new svgtiny_resolve_url() function

src/svgtiny_css.c: new file with new svgtiny_resolve_url() function

Define a stub relative-URL resolver that assumes all URLs are absolute
and returns them unmodified. Some method of doing this is required by
libcss. At the moment, libcss does not support any SVG properties that
take url() values, so not much is lost.