--video-elements

--video-elements is where "deinterlace mode=interlaced" can go if needed.
or any other string of gst elements:
defaults to: --video-elements = "videoconvert ! videorate ! videoscale"

added --no-clock - don't use voctocore clock (use gst's default clock.)

use mktemp to mush a bug:
#927347: voctomix-outcasts: don't use hardcoded filenames in /tmp - even for testing