DrawList: consistently support scope flags (ImDrawFlags::Flags) + primitive flags.

ImDrawFlags_AAEnds and ImDrawFlags_StrokeLegacy ok. (the most important ones).
ImDrawFlags_UseTexForRoundCorners ok.
ImDrawFlags_AAFill, ImDrawFlags_AAStrokes: for primitives: only for selected ones taking flags params. Haven't added the flag to all functions because it feels like a fringe use case.
ImDrawFlags_TextNoPixelSnap supported in RenderText(), per-prim (9417)