Multi-render

Upstream PR

Title: Allow render to be called multiple times for one BeginFrame

Link: https://github.com/flutter/engine/pull/36438

One result of the code change is that, it allows multi-render in one plain frame.

It is a must to add that change (create a continuation if there is none) to Render. This is because, we call Render multiple times for one BeginFrame. The original code will reject all Renders except for the first one, thus the whole flutter_smooth will not work because we can no longer submit anything more to render.