src/svgtiny.c: parse styles in svgtiny_parse_paint_attributes()

src/svgtiny.c: parse styles in svgtiny_parse_paint_attributes()

Now that the scaffolding is in place, we do the work to parse our two
supported CSS paint attributes, fill-opacity and stroke-opacity. This
amounts to,

  * Calling svgtiny_select_style(), which is just a wrapper around
    css_select_style()
  * Passing the result to css_computed_foo_opacity() to get the
    computed styles
  * Populating fill_opacity and stroke_opacity in the parser state
    if the corresponding styles exist

There's also a temporary hack to avoid crashing on the root node that
later goes away when we implement parent/child style composition.