Currently, this only affects untagged RGB/XYZ/Gray, which get forced to
their corresponding metadata before entering the filter graph. The main
justification for this change, however, is the planned ability to add
automatic promotion of unspecified yuv to mpeg range yuv.
Notably, this change will never allow accidentally cross-promoting
unspecified to jpeg or to a specific YUV matrix, since that is still
bound by the constraints of YUV range negotiation as set up by
query_formats.
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