mkv: make sure the first cluster timecode is 'close to zero'.

when recording to disk, the subscription is started ~30s before the
scheduled event actually starts. This causes the pts time stamps to have an
offset of about 30s. This patch moves the start time filter condition
from the end of the pipeline to be a configurable property of the tsfix pipe.
dvr: rearrange the pipes so that globalheaders gets the packets before tsfix (and the start time filter).