deinterlace: Drain the field history if the caps are changing
deinterlace: Drain the field history if the caps are changing

Otherwise we will use fields from the old caps with everything set up for the
new caps, causing crashes and worse.

Also don't do anything if the same caps are set twice.

GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good