Crossfades keep animated scenes on screen, and frames from any production

Fixed

• Crossfades keep moving scenes on screen. When a dissolve led into or out of a scene with animated characters or drawings, that scene was missing from the fade: the picture faded through an empty frame and the scene popped in at the cut. Both scenes now stay visible for the whole crossfade.

• Frame previews work on any production. An agent asking for frames of a finished production with frame_render got a "has no measured duration" error unless that production was the one open in the app. Frames now render from the production the agent names.