Layout columns for the frame. This is in essence the followup to BeginTable(). Runs on the first call to TableNextRow(), to give a chance for TableSetupColumn() to be called first. FIXME-TABLE: Our width (and therefore our WorkRect) will be minimal in the first frame for _WidthAuto columns. Increase feedback side-effect with widgets relying on WorkRect.Max.x... Maybe provide a default distribution
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nothing calls this directly
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