Still image detection is inaccurate about olive HOT 5 CLOSED

This issue also occurs with "older" formats such as Bink and EA MAD

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Olive also suffers from duration detection issues if a container (particularly older ones) doesn't contain a duration value. Heuristics will have to be mocked up to detect these things if they are not available.

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(avg_frame_rate = nan) is our best statistic
(avg_frame_rate = nan + duration = -9223372036854775808) is almost definitely an image
some image types like jpegs actually a duration, but they still don't have a avg_frame_rate
current conclusion: all images have (avg_frame_rate = nan)

but some video formats don't contain durations, and for these we'll need to discover the frame rate separately. luckily we already have a thread going through the entire file for thumbnail/waveform generation so we should be able to latch onto this (and combine it with #85 so the file can only be used one its duration is known)

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biggest question is: do image sequences report (avg_frame_rate = nan)

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Technically, using this heuristic has fixed this problem. So, that's cool

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