deinterlace: use output caps to compute buffer size
deinterlace: use output caps to compute buffer size

In interlace-mode=alternate the input buffers have half the size of the
output ones as each field has its own buffer.

pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good