Defer demuxer pause until buffers full to fix exit after pause

This change fixes the following issue (closed but never resolved):

http://trac.xbmc.org/ticket/11122

Previously, the source stream would be paused immediately. If the video/audio buffers were not full at the time, CDVDPlayer would continue to call m_pDemuxer->Read(). In some cases this can then block indefinitely (CDVDInputStreamRTMP for example).