setResult existed and nothing called it; the timeline already drew
scene.result. One wire short of a loop -- generate a scene, and what came
back is on the strip and still there after a reload.
The scene id is read when the run STARTS. A run takes minutes and the user
goes on clicking, so the selected scene when it finishes is not the one that
was generated.
A strip cell now falls back to its glyph when a thumbnail does not arrive,
the way the bin's cells already did -- the browser's broken-image icon reads
as a damaged file rather than a missing one.