add seed + autoupdate duration

[cocktailpeanut]

1. Added seed (by default 0, which randomizes the seed every time)
2. Duration maximum value gets auto-updated whenever you switch between 768p and 384p (31 and 16, respectively). Note that the default value is always set to 16 since the 31 frame option might require too much resources for certain machines while 16 works fine.

Here's what it looks like:

[feifeiobama]

Thank you for the commit!