Fallback to onefield deint rather than none for non-1x speeds

[doug@…]

If bob deinterlacing is turned on, current Mythfrontend turns it off if the playback speed is not 1x (such as when fast-forward/rewinding or using timestretch). This was done because otherwise panning motions can be quite unsmooth, particularly with timestretch. Unfortunately turning off all deinterlacing results in the usual ugly interlaced artifacts, particularly if the output is scaled (e.g. 1080i material being displayed at 1280x720).

This patch uses the onefield deinterlacer instead for non 1x. I chose onefield because it's the lowest resource-consumption deinterlacer, and if you're fast forwarding you could be processing frames much faster than usual.

[doug@…]

The patch

[danielk]

(In [6723]) Closes #15.

This applies Doug's patch for nicer looking video at speeds other than 1x. It uses one field deinterlace instead of no-deinterlace at these speeds.