[RFC/bluray] Attempt to fix ext. subtitle and language issues with blurays

@elupus, @vicbitter
I suspect this isn't a proper solution, but I wanted to present my findings.

The inability to retrieve the language if title is played directly, i.e. not over the play menu button, was introduced here: f55d9dca1495c8bf478c026e41bbf42da7388ac0. The problem is that 
```m_clip  = (uint32_t)-1;``` isn't updated in that case.

The issue with external subs is due to a "wrong"/not happening  timestamp correction:
```c++
  if(dynamic_cast<CDVDInputStream::IMenus*>(m_pInput))
    starttime = dynamic_cast<CDVDInputStream::IMenus*>(m_pInput)->GetTimeStampCorrection() / DVD_TIME_BASE;
  else if (m_pFormatContext->start_time != (int64_t)AV_NOPTS_VALUE)
    starttime = (double)m_pFormatContext->start_time / AV_TIME_BASE;
```
because ```GetTimeStampCorrection()``` always returns ```0``` for blurays.