src/svgtiny.c: deallocate stylesheets before destroying their context

src/svgtiny.c: deallocate stylesheets before destroying their context

We allocate stylesheets in svgtiny_parse_style_element() and append
them to our global select_ctx, but the select_ctx retains only a const
reference to them and is not responsible for freeing their resources.
Instead, we have to do it, right before we destroy the context itself.

This is a little ugly because we have to de-const the sheet references
before we can destroy them. This relies on the non-local knowledge
that every sheet appended to the context does in fact originate in the
one function svgtiny_parse_style_element(). On the other hand, using
the context to keep track of these sheets saves us the trouble of
maintaining a duplicate array that would differ only in type
signature.