v4l2bufferpool: Don't mark jpeg frames as deltas
v4l2bufferpool: Don't mark jpeg frames as deltas

JPEG formats are encoded, but they never have keyframe flag. But in
fact they are keyframes

https://bugzilla.gnome.org/show_bug.cgi?id=785990

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