Internally most display module use vd->source rather than a variable to use the input format.
If vout_SetDisplayFormat() succeeds the new format must be in vd->source. On failure vd->source will use the old (chroma) value agreed upon and we'll still provide pictures in the format that was agreed upon.
This is the same as when opening a display module. We no longer provide the source format, the display module relies on vd->source.