A setting driven by a flow re-renders whatever reads it on every message.
`LookProvider` and `PaletteProvider` built a fresh context value each time,
and `CanvasSurface` called its render prop again, so a value on `theme`,
`palette`, `look`, `background`, `touch` or `locked` reconciled the whole
widget tree and re-ran every chart's `setData`.
The palette is now read as roles once per set of colours, the tokens and
both context values are memoised on it, and the canvas holds the tiles it
built while the callback and the scale stay put. `WidgetBody` is memoised
too: its two props are stable while the document is, so a grid rendered for
its own reasons no longer rebuilds every chart with it.
stroblme
46f9575870