VC1testenc: convert pts values to correct time-base.
VC1 test container always uses time-base 1 ms, so we must convert
from whatever time-base the application gave us to that, otherwise
the video will play at ridiculous speeds.
It would be possible to signal that a container supports only one
time-base and have code in a layer above do the conversion, but
for a single format this seems over-engineered.