videoflip: Missing support for ARGB64 video

GStreamer version: 1.16.2.

Operating system: Windows 8.1 x64 (desktop), macOS 10.14.6.

Reproduce: run the following command (substitute the real video file path):

gst-launch-1.0 uridecodebin uri="file:///path/to/movie" ! videoconvert ! video/x-raw, format=ARGB64 ! videoflip video-direction=vert ! videoconvert ! pngenc ! multifilesink max-files=1 location=gst_dec_%05d.png

Expected: a flipped frame from the video as a PNG file.

Actual: no PNG file, a stderr message instead: WARNING: erroneous pipeline: could not link videoconvert0 to videoflip0, videoflip0 can't handle caps video/x-raw, format=(string)ARGB64.

Note: if we replace ARGB64 with RGBA, the pipeline works as expected.