WPXSvInputStreamImpl was hammering on getFilePos pretty hard, and
getFilePos uses a mutex, which is slow when it is called from every
single read. So switch to using std::atomic to access position.

This is specifically fixing the performance of queryTypeByDescriptor
when called from a basic macro on a local test file.

This takes my test macro from 8s to 4s.

Change-Id: Iab707a374359e2ee0e92425b2d9a903d67cb53d4
Reviewed-on: https://gerrit.libreoffice.org/73377
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>