Frame-list animations have no per-frame duration #22

A frames: [...] animation has one duration that applies to every frame.
gif.rs::render_gif takes a single duration_ms, and there's no per-frame
timing field (frame_metadata only carries hitboxes). Speech timing and idle
holds need uneven frame lengths. Workaround: a hold is the same
composition repeated (skull_talking repeats skull for its pauses). Nothing
is lost in the GIF, but the frame lists get long.

Found while animating examples/skull (#21).

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