Should processEvents() call R_ProcessEvents() on all platforms?

[kevinushey]

On non-Apple Unix platforms, we check and call input handlers, but we don't ever actually invoke R_ProcessEvents():

rstudio/src/cpp/r/session/REmbeddedPosix.cpp

Lines 229 to 247 in 5860de7

	void processEvents()
	{
	#ifdef __APPLE__
	R_ProcessEvents();
	// pickup X11 graphics device events (if any) via X11 input handler
	fd_set* what = R_checkActivity(0,1);
	if (what != nullptr)
	R_runHandlers(R_InputHandlers, what);
	#else
	// check for activity on standard input handlers (but ignore stdin).
	// return immediately if there is no input currently available
	fd_set* what = R_checkActivity(0,1);
	// run handlers on the input (or run the polled event handler if there
	// is no input currently available)
	R_runHandlers(R_InputHandlers, what);
	#endif
	}

In practice, it seems like this isn't an issue because R_ProcessEvents() calls the function pointer ptr_R_ProcessEvents, which only gets set on macOS for the Quartz device. However, conceptually we likely should be calling R_ProcessEvents() regardless.