src/svgtiny.c: add impotent svgtiny_preparse_styles() function

src/svgtiny.c: add impotent svgtiny_preparse_styles() function

We declare and add the svgtiny_preparse_styles() function that will be
used to make a first pass through the SVG document and parse all of
the <style> elements. At the moment it is only half implemented:
once we find a <style> element, we do nothing with it. A separate
function will be used to parse a <style> once we have it.