🐛 Fix deinterlace crash in v4l2src when pixel format is not raw video

The deinterlace Gstreamer element embedded in the v4l2src pipeline only accepts pixel formats that are raw video. When it receives a video stream that is not raw video (such as H264 streams output by some webcams) it cannot negotiate caps and therefore, crashes.

This fix checks if the incoming stream is raw video; if it's not, the deinterlace element is not added to the pipeline.