META_VideoCrop is present on every buffer negotiated through the
adapter, even when the producer never sets a meaningful crop
region. Before commit c634ef961, gst_buffer_get_video_crop_meta
returned NULL on new GstBuffers so the zero values were never
applied. After that commit, gst_buffer_add_video_crop_meta always
succeeds, and an invalid crop produces black frames.
Without this, the following produces black frames,
Producer:
gst-launch-1.0 videotestsrc is-live=true ! \
video/x-raw,format=I420,width=1280,height=720,framerate=24/1 ! \
pipewiresink mode=provide \
stream-properties="properties,media.class=Video/Source,media.role=Camera" \
client-name=VirtualCam
Consumer:
gst-launch-1.0 pipewiresrc path=<id> ! videoconvert ! autovideosink