movenc: Always write EDTS when we are able to.

[ffmpeg] / tests / ref / vsynth2 / ffv1

d72b0960e162d4998b9acbabb07e99ab *./tests/data/vsynth2/ffv1.avi
3525804 ./tests/data/vsynth2/ffv1.avi
dde5895817ad9d219f79a52d0bdfb001 *./tests/data/ffv1.vsynth2.out.yuv
stddev:    0.00 PSNR:999.99 MAXDIFF:    0 bytes:  7603200/  7603200