vout: make vd->source the new input format on update_format

Open Steve Lhomme requested to merge robUx4/vlc:update_format-source into master

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.

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