src/svgtiny_css.c: new function svgtiny_create_stylesheet()

src/svgtiny_css.c: new function svgtiny_create_stylesheet()

We add a function to handle the creation of a new stylesheet with the
default set of parameters. This is in preparation for parsing the
inline styles, which a priori would involve copy/pasting a lot of

  params.foo = bar;

lines from svgtiny_parse_style_element(). The goal is to factor the
common bits out of both the <style> and style= implementations.