h265parse: Detect height change on field-based interlaced files

h265parse: Detect height change on field-based interlaced files

The first time update_src_caps is called, there's no frame parsed yet,
therefore we don't know whether the file has alternate-field interlacing
mode. If we run it again after we have a frame, it might be that now we
have the SEI pic_struct parsed, and therefore we know that it's
field-based interlaced, and therefore the height must be multiplied by
two. Earlier on this was not detected as a change.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2022>