* gst_ffmpegviddec_frame() is the only caller of gst_ffmpegviddec_video_frame()
and has the same signature. Just move the checks into a single function and
use that.
* Make it clear which frames are the input and output ones in
gst_ffmpegviddec_video_frame() to make issues like the one fixed in the previous
commit more obvious.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6851>
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