WinRT TTS: Improve AudioSource buffer management

Replace the raw byte pointer + m_bufferOffset approach with a
QSpan<const char> member that is shrunk as data is consumed.
This removes m_bufferOffset, m_riffHeaderChecked, and the
bufferByteAccess::Buffer() call on every readData invocation.

The RIFF header skip moves into BytesReadyHandler Invoke, using
m_bytesRead == 0 as a natural first-buffer guard.

Fix a bug where the byte-position pause path added a spurious +44
(a stale WaveHeaderLength constant) to the truncation point,
overshooting the requested pause boundary by ~1.4 ms.

Pick-to: 6.12
Change-Id: I9b67841a686bf353b839cbe41015835a91272544
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>