lavc/htmlsubtitles: improve handling broken garbage

This commit switches off forced correct nesting of tags and only keeps
it for font tags. See long explanations in the code for the rationale.

This results in various FATE changes which I'll explain here:

- various swapping in font attributes, this is mostly noise due to the
  old reverse stack way of printing them. The new one is more correct as
  the last attribute takes over the previous ones.

- unrecognized tags disappears

- invalid tags that were previously displayed aren't anymore (instead,
  we have a warning). This is better for the end user

The main benefit of this commit is to be more tolerant to error, leading
to a better handling of badly nested tags or random wrong formatting for
the end user.
