src/svgtiny_parse.c: add svgtiny_parse_style_inline() function

src/svgtiny_parse.c: add svgtiny_parse_style_inline() function

This function parses the contents of an inline style="..." attribute
into a libcss stylesheet, or NULL if anything went wrong.

We use the new function in svgtiny_parse_paint_attributes() to parse
an inline sheet... and then (for now) do nothing with it. We are able
to parse the inline style="..." attributes right now but more
scaffolding is needed before we can utilize css_select_style().