FIXME: vtdec: Support outputting ARGB64 hackily

kCVPixelFormatType_64ARGB is actually BE, but GST_VIDEO_FORMAT_ARGB64
is LE, so we need to byte-swap each component manually. This is very
hacky, and the correct solution is to add GST_VIDEO_FORMAT_ARGB64_BE
and add converters for it in videoconvert.

Likely due to this byte-swapping, we black frames when outputting to
glimagesink, so disable gl features when outputting ARGB64.

This is the same thing that is done in the old ProRes decoder element,
so it should be fine for now.
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