avviddec: Rename variables and fuse function
- 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.