Allow more than 1 active subtitle/multi-line subs, and formatting

[MrSapps]

Currently if more than one sub title is active it won't render:

                // TODO: Render all active subs, not just the first one
                current_subs = subs[0]->Text().c_str();
                if (subs.size() > 1)
                {
                    LOG_WARNING("Too many active subtitles " << subs.size());
                }

And if a subtitle spans more than 1 line its draw as 1 line - i.e \n is ignored.

Additionally basic formatting such as [i]test[/i] should work. Probably at least i,b and u.

[MrSapps]

This is half done now, [i]test[/i] [b]test[/b] works. But displaying more than one sub does not.