Handle dts properly

This PR is the consequence of discussions in #32 and #46,

The fixdts3 branch is an update of fixdts2 which limits the number of decoding loops.
As a reminder it will : 
  * enable to call VPU_DecDecodeBuf more than 2 times per  ::Decode call.
  * returns dts instead of NOPTS when no pts is available

(more details are available in this [comment](https://github.com/xbmc-imx6/xbmc/pull/46#issuecomment-38121275))