dillon1000/react

[Scheduler] requestPaint (#15960)

* [Scheduler] requestPaint

Signals to Scheduler that the browser needs to paint the screen. React
will call it in the commit phase. Scheduler will yield at the end of
the current frame, even if there is no pending input.

When `isInputPending` is not available, this has no effect, because we
yield at the end of every frame regardless.

React will call `requestPaint` in the commit phase as long as there's at
least one effect. We could choose not to call it if none of the effects
are DOM mutations, but this is so rare that it doesn't seem worthwhile
to bother checking.

* Fall back gracefully if requestPaint is missing