Only if the uppx increases by more then 2 we redraw the entire line
otherwise just ds with previous params and update the current curve.
This *should* avoid strange lower sample rate artefacts from showing on
updates.
Summary:
- stash both uppx and px width in `._dsi` (downsample info)
- use the new `ohlc_to_m4_line()` flags
- add notes about using `.reserve()` and friends
- always delete last `._array` ref prior to line updates
561d7e0349