Notify backends of updates to plot render settings

With #7247 we now expect backends to send pre-renderings of plots in comm_open messages. This makes the plots feel snappier as a version of new plots is displayed right away in the plot pane (see #5184). However, the experience is currently subpar for applications that run in the console for a long time while generating plots. In that case, all plots created while the area geometry is changing will use outdated render settings, causing distorted output.

This happens because the only time a backend can currently update the current render settings is during a render request. If the pre-rendering was not generated with up-to-date settings, the frontend sends a request right away. So in theory, the backend would soon be notified of the updated settings. However, since these render requests are comm messages queued on Shell, they must wait for all current and pending execute requests to finish before being handled by the backend. This is why long-running applications that generate plots do not behave properly: the updated settings are only received once the application has stopped.

Concretely, it should be possible to run this loop, change the plot pane size while the plots are being rendered, and get proper pre-renderings right away:

for (i in 1:20) {
  plot(i)
  Sys.sleep(1)
}

Also relevant for things like demo(graphics): Currently when the second plot is opens, the plot history strip appears and changes the plot render area. Subsequent plots are rendered with outdated settings.

To fix this:

• We should send events/notifications to backends with updated render settings as soon as the plot pane area changes (now sent as requests but should become notifications, see #7448).

• These notifications should be sent asynchronously (#7447). While an expression is running, we can't update the backend via a regular Shell message so all the following plots are rendered a little off and are only rerendered once the console stops being busy.