Fix a problem where the previews would flicker.

Fix a problem where the previews would flicker.

When setting up a new frame, we'd set LiveInputWrapper state
(ie., which textures should FlatInput girender from) for all
chains, even preview chains which were just about to get rendered.
Those could then render from the wrong texture number -- and worse,
that texture was still in the process of being uploaded, causing
corrupted display.

There are probably still concurrency bugs left here, but this was
the biggest and most annoying one.