src/svgtiny_internal.h: add CSS context to parser state

src/svgtiny_internal.h: add CSS context to parser state

Add a css_select_ctx pointer to the svgtiny_parse_state that is
threaded through each phase of SVG parsing. This will allow us to
populate the context with any <style> sheets we find; later, we can
consult those stylesheets while constructing our svgtiny_diagram (the
abstract representation into which we parse an SVG).