matroskademux: should be able to just change rate when streaming

Submitted by Nicola @drakkan

Link to original bug (#696692)

Description

Actually a streamable file, produced for example with mastroskamux streamable=true, cannot be played at rate different from 1x (setting sync=false on videosink play at max speed but not 2x, 4x ecc.), I understand that seek cannot work for such files, but fast forward could be implemented, I'm using a quick workaround in my app that use appsink/appsrc, maybe sometime in future I'll try to implement this inside some demuxer (for example matroskademux). This bug report is for keep track of this feature and for share implementation hints