Changed the OSS backend to detect if the DSP requires a conversion of

Changed the OSS backend to detect if the DSP requires a conversion of
the sampling data in order to play it.  Currently it fails if this is
the case but in the future a conversion could be done, or it could
fall-back to another backend that is able to handle it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@26179 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775