Render phase can be interrupted

[jstarry]

Problem

In #1309, the scheduler was modified to group render tasks together to improve performance. However, grouped render tasks can get disrupted if any of the associated components has a rendered() lifecycle method that pushes an update task into the scheduler (self.link.send_message(..)).

Proposed Changes

Split the scheduler's "render" task queue into "render" and "rendered" so that renders won't be disrupted and can run in batches properly