src/svgtiny_css.c: case-insensitivity for node_has_attribute_substring()

src/svgtiny_css.c: case-insensitivity for node_has_attribute_substring()

All of the CSS attribute selectors take a flag to make the comparison
case-insensitive. Libcss doesn't support that yet, but in the case of
node_has_attribute_substring(), factoring out the implementation and
letting it take an "insensitive" flag will be of use to us when we
implement the node_is_lang() select handler.