In response to Coverity CID 383367 - it is perhaps false positive but
there has been another small bug - the step should be length of to, not
from (which may be longer).
+ written a test for it
Set signal as progressive (not-PsF) by default - previously the value
was not touched which may be a bit misleading when DeckLink outputs
actually interlaced signal.
Issue warning that the option is set and keep user opt out from setting
this value.
