Unnecessary data saving of first pass when 2 pass rendering is selected

Currently when rendering in 2 pass mode melt is called two times. The output video of the first pass is dropped at start of second pass and replaced with the final video data.

One can avoid a lot of HDD writing by redirecting the video from the first pass to /dev/null. Currently I am doing this by hand, but it might be easy to implement for kdenlive_render.

There is also a second minor point with skripting of the render process. The generated shell scripts create a SOURCE and TARGET variable but never use them. Instead this data is hardcoded in the PARAMETERS variable:

#! /bin/sh

PARAMETERS="dv_pal avformat - /home/flo/kdenlive/scripts/full.sh.mlt /tmp/1.vob f=dvd vcodec=mpeg2video acodec=ac3 b=5000k maxrate=8000k minrate=0 bufsize=1835008 mux_packet_s=2048 mux_rate=10080000 ab=192k ar=48000 s=720x576 g=15 me_range=63 trellis=1 profile=dv_pal pass=2"